OBDII GENERIC PID DIAGNOSIS BY KARL SEYFERT
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ome scan tools call it the global OBD II mode, while others describe it as the OBD II generic mode. The OBD II generic mode allows a technician to attach his scan tool to an OBD II-compliant vehicle and begin collecting data without entering any VIN information into the scan tool. You may need to specifically select “OBD II Generic” from the scan tool menu. Some scan tools may need a software module or personality key before they’ll work in generic OBD II test mode. The original list of generic data parameters mandated by OBD II and described in SAE J1979 was short and designed to provide critical system data only. The useful types of data we can retrieve from OBD II generic include
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short-term and long-term fuel trim values, oxygen sensor voltages, engine and intake air temperatures, MAF or MAP values, rpm, calculated load, spark timing and diagnostic trouble code (DTC) count. Freeze frame data and readiness status also are available in OBD II generic mode. A generic scan tool also should be able to erase trouble codes and freeze frame data when commanded to do so. Data coming to the scan tool through the mandated OBD II generic interface may not arrive as fast as data sent over one of the dedicated data link connector (DLC) terminals. The vehicle manufacturer has the option of using a faster data transfer speed on other DLC pins. Data on the generic interface also may not be as complete as the information you’ll get on many manufacturer-
Photo: Karl Seyfert
A wealth of diagnostic information is available on late-model OBD II-compliant vehicles, even when ‘enhanced’ or ‘manufacturer-specific’ PIDs are not accessible. It doesn’t take much to use this information to its best advantage.
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Photos: Karl Seyfert
OBD II GENERIC PID DIAGNOSIS
Here’s a basic scanner display showing OBD II generic PIDs. Slow-changing PIDs like IAT and ECT can be followed fairly easily in this format, but it’s difficult to spot glitches in faster moving PIDs like Spark Advance.
Mode 1: Show current data Mode 2: Show freeze frame data Mode 3: Show stored trouble codes Mode 4: Clear trouble codes and stored values Mode 5: Test results, oxygen sensors Mode 6: Test results, noncontinuously monitored Mode 7: Show pending trouble codes Mode 8: Special control mode Mode 9: Request vehicle information Modes 1 and 2 are basically identical. Mode 1 provides current information, Mode 2 a snapshot of the same data taken at the point when the last diagnostic trouble code was set. The exceptions are PID 01, which is available only Photo courtesy Snap-on Diagnostics
specific or enhanced interfaces. For example, you may see an engine coolant temperature (ECT) value in degrees on the OBD II generic parameter identification (PID) list. A manufacturerspecific data list may display ECT status in Fahrenheit or Celsius and add a separate PID for the ECT signal voltage. In spite of these and other limitations, OBD II generic mode still contains many of the trouble codes, freeze frame data and basic datastream information needed to solve many emissions-related issues. There are nine modes of operation described in the original J1979 OBD II standard. They are:
This scan tool also allows the user to graph some PIDs, while continuing to display the others in conventional numeric format. Due to OBD II’s refresh capabilities on some vehicles, it’s best to limit your PID choices to those directly related to your diagnostic approach.
This photo illustrates how far PID data collection and display have come. Several hundred thousand techs are still using the original Snap-on “brick” (on the left), which displays a limited amount of PID data on its screen. Scrolling up or down revealed more PIDs. The color version on the right brought graphing capability to the brick, and extended the product’s life span by several years.
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in Mode 1, and PID 02, available only in Mode 2. If Mode 2 PID 02 returns zero, then there’s no snapshot and all other Mode 2 data is meaningless. Vehicle manufacturers are not required to support all modes. Each manufacturer may define additional modes above Mode 9 for other information. Most vehicles from the J1979 era supported 13 to 20 parameters. The recent phase-in of new parameters will make OBD II generic data even more valuable. The California Air Resources Board (CARB) revisions to OBD II CAN-equipped vehicles have increased the number of potential generic parameters to more than a hundred. Not all vehicles will support all PIDs, and there are many manufacturer-defined PIDs that are not included in the OBD II standard. Even so, the quality and quantity of data have increased significantly. For more information on the new PIDs that were added to 2004 and later CAN-equipped vehicles, refer to Bob Pattengale’s article “Interpreting Generic Scan Data” in the March 2005 issue of MOTOR. A PDF copy of the article can be downloaded at www.motor.com.
Establish a Baseline If you’re repairing a vehicle that has stored one or more DTCs, make sure you collect the freeze frame data before erasing the stored codes. This data can
OBD II GENERIC PID DIAGNOSIS
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Photo courtesy Injectoclean
Photo courtesy SPX/OTC
Screen capture: Jorge Menchu
be used for comparison after IAC counts look too high or your repairs. The “before” too low? Compare data items freeze frame shot and its PID to known-good values you’d data establish the baseline. expect to see for similar opAs you begin your diagnoerating conditions on similar sis, correct basic problems vehicles. first—loose belts, weak batCheck short-term fuel teries, corroded cables, low trim (STFT) and long-term coolant levels and the like. fuel trim (LTFT). Fuel trim The battery and charging sysis a key diagnostic parameter tem are especially important, and tells you what the comdue to their effect on vehicle puter is doing to control fuel electronics. A good battery, a delivery and how the adapproperly functioning alternative strategy is operating. tor and good connections at STFT and LTFT are expower and ground circuits pressed as a percentage, with are essential. You can’t asthe ideal range being within The Snap-on MODIS is a combination scanner, lab/ignisume that OBD II will detect tion scope, DVOM and Troubleshooter. In scanner mode, ±5%. Positive fuel trim pera voltage supply problem that MODIS can graph several parameters simultaneously, centages indicate that the can affect the entire system. as seen in this screen capture. Remember, although powertrain control module If you have an intermittent these may look like scope patterns, the reporting rate (PCM) is attempting to enproblem that comes and for PID data on a scanner isn’t nearly as fast. richen the fuel mixture to goes, or random problems compensate for a perceived that don’t follow a logical pattern, check down the battery voltage and the results lean condition. Negative fuel trim perthe grounds for the PCM and any other of any simple tests, such as fuel pressure centages indicate that the PCM is atcontroller in the vehicle. or engine vacuum. Look at the Readi- tempting to enlean the fuel mixture to If the basics check out, focus your di- ness Status display to see if there are compensate for a perceived rich condiagnosis on critical engine parameters any monitors that aren’t running to tion. STFT will normally sweep rapidly and sensors first. Write down what you completion. between enrichment and enleanment, find; there’s too much information to while LTFT will remain more stable. If keep it all in your head. Add any infor- Datastream Analysis either STFT or LTFT exceeds ±10%, mation collected from the vehicle own- Take your time when you begin looking this should alert you to a potential er regarding vehicle performance. Jot at the live OBD II datastream. If you se- problem. lect too many items at one time, the scan tool update will slow. The more PIDs you select, the slower the update rate will be. Look carefully at the PIDs and their values. Is there one line of data that seems wrong? Compare data items to one another. Do MAP and BARO agree key on, engine off (KOEO)? Are IAT and ECT the same when the engine is cold KOEO? The ECT and IAT should be within 5°F of each other. ECT should reach operating temperature, preferably 190°F or higher. If the ECT is too low, the PCM may richen the fuel mixture to compensate for a (perceived) coldengine condition. IAT should read ambient temperature or close to underhood When scan tool screen real estate is temperature, depending on the location limited, porting the scan tool into a laptop or desktop PC allows you to of the sensor. graph more PIDs simultaneously. An on-screen description of the PID Is the battery voltage good KOEO? The PC’s much larger memory cadisplayed below the graphing data Is the charging voltage adequate when pacity also makes it possible to colmay help you to understand what lect PID data in movie format for you’re looking at, and avoid misunder- the engine starts? Do the MAP and BARO readings seem logical? Do the later playback and analysis. standings with measurement units.
Screen captures: Jorge Menchu
OBD II GENERIC PID DIAGNOSIS
Graphs aren’t the only way to display PID data. Once transferred to the PC with its greater screen real estate, PID data can be converted to formats that relate to the data. A red thermometer scale is much easier to follow than changing numbers on a scan tool.
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The PCM uses this information to calculate the amount of fuel that should be delivered to achieve the desired air/fuel mixture. Check the MAF sensor for accuracy in various rpm ranges, including wide-open throttle (WOT), and compare it with the manufacturer’s recommendations. When checking MAF sensor read-
Screen capture courtesy Bosch Diagnostics
Determine if the condition exists in more than one operating range. Check fuel trim at idle, at 1500 rpm and at 2500 rpm. If LTFT B1 is 20% at idle but corrects to 5% at both 1500 and 2500 rpm, focus your diagnosis on factors that can cause a lean condition at idle, such as a vacuum leak. If the condition exists in all rpm ranges, the cause is more likely to be fuel-related, such as a bad fuel pump, restricted injectors, etc. Fuel trim can also be used to identify which bank of cylinders is causing a problem on bank-to-bank fuel control engines. For example, if LTFT B1 is 25% and LTFT B2 is 5%, the source of the problem is associated with B1 cylinders only, and your diagnosis should focus on factors related to B1 cylinders only. The following parameters could affect fuel trim or provide additional diagnostic information. Also, even if fuel trim is not a concern, you might find an indication of another problem when reviewing these parameters: Fuel System 1 Status and Fuel System 2 Status should be in closed-loop (CL). If the PCM is not able to achieve CL, the fuel trim data may not be accurate. If the system includes one, the mass airflow (MAF) sensor measures the amount of air flowing into the engine.
PC-based scan tools excel at capturing and displaying large amounts of PID data for later analysis. Graphing the data, then analyzing it on-screen, may allow you to spot inconsistencies and provides an easy method for overlaying similar or related PID data.
Here’s a peek at some of the additional PID data that’s available on latemodel vehicles. This screen capture was taken from a CAN-enabled 2005 vehicle, and includes PIDs for EVAP PURGE, FUEL LEVEL and WARM-UPS, as well as familiar PIDs like BARO. This much PID data in generic mode should aid in diagnosis when manufacturerspecific PID data is not available.
ings, be sure to identify the unit of measurement. The scan tool may report the information in grams per second (gm/S) or pounds per minute (lb/min). Some technicians replace the sensor, only to realize later that the scan tool was not set correctly. Some scan tools let you change the units of measurement for different PIDs so the scan tool matches the specification in your reference manual. Most scan tools let you switch easily between Fahrenheit and Celsius temperature scales, for example. But MAF specs can be confusing when the scan tool shows lb/min and we have a spec for gm/S. Here are a few common conversion formulas, in case your scan tool doesn’t support all of these units of measurement: Degrees Fahrenheit 32 5/9 Degrees Celsius Degrees Celsius 9/5 + 32 Degrees Fahrenheit lb/min 7.5 gm/S gm/S 1.32 lb/min The Manifold Absolute Pressure (MAP) Sensor PID, if available, indicates manifold pressure, which is used by the PCM to calculate engine load. The reading is normally displayed in inches of mercury (in./Hg). Don’t confuse the MAP sensor parameter with intake manifold vacuum; they’re not the same. Use this formula: barometric
pressure (BARO) MAP intake manifold vacuum. For example, BARO (27.5 in./Hg) MAP (10.5) intake manifold vacuum (17.0 in./Hg). Some vehicles are equipped with only a MAF sensor, some have only a MAP sensor and some are equipped with both. The PIDs for Oxygen Sensor Output Voltage B1S1, B2S1, B1S2, etc., are used by the PCM to control fuel mixture and to detect catalytic converter degradation. The scan tool can be used to check basic sensor operation. The sensor must exceed .8 volt and drop below .2 volt, and the transition from low to high and high to low should be quick. A good snap throttle test will verify the sensor’s ability to achieve the .8 and .2 voltage limits. If this method doesn’t work, use a bottle of propane to manually richen the fuel mixture to check the oxygen sensor’s maximum voltage output. To check the sensor’s low voltage range, simply create a lean condition and check the voltage.
Remember, your scan tool is not a lab scope. You’re not measuring the sensor in real time. The PCM receives the data from the oxygen sensor, processes it, then reports it to the scan tool. Also, a fundamental OBD II generic limitation is the speed at which that data is delivered to the scan tool. In most cases, the fastest possible data rate is approximately 10 times a second, with only one parameter selected. If you’re requesting and/or displaying 10 parameters, this slows the data sample rate, and each parameter is reported to the scan tool just once per second. You can achieve the best results by graphing or displaying data from each oxygen sensor separately. If the transition seems slow, the sensor should be tested with a lab scope to verify the diagnosis before you replace it. The Engine Speed (RPM) and Ignition Timing Advance PIDs can be used to verify good idle control strategy. Again, these are best checked using a graphing scan tool. Check the RPM,
Vehicle Speed Sensor (VSS) and Throttle Position Sensor (TPS) PIDs for accuracy. These parameters can also be used as reference points to duplicate symptoms and locate problems in recordings. Most PID values can be verified by a voltage, frequency, temperature, vacuum or pressure test. Engine coolant temperature, for example, can be verified with a noncontact temperature tester, while intake manifold vacuum can be verified with an accurate vacuum gauge. Electrical values also should be tested with a DVOM. If the electrical value exists at the sensor but not at the appropriate PCM terminal, then the component might be experiencing a circuit fault.
Calculated Values Calculated scan tool values can cause a lot of confusion. The PCM may detect a failed ECT sensor or circuit and store a DTC. Without the ECT sensor input,
Circle #31
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OBD II GENERIC PID DIAGNOSIS the PCM has no idea what the coolant temperature really is, so it may “plug in” a temperature it thinks will work to keep the engine running long enough to get it to a repair shop. When it does this, your scanner will display the failsafe value. You might think it’s a live value from a working sensor, when it isn’t.
Also be aware that when a component such as an oxygen sensor is disconnected, the PCM may substitute a default value into the datastream displayed on the scan tool. If a PID is static and doesn’t track with engine operating conditions, it may be a default value that merits further investigation.
Circle #32
Circle #33
Circle #35
Circle #34
Graphing Data If you’ve ever found it difficult to compare several parameters at once on a small scan tool screen, graphing PIDs is an appealing proposition. Graphing multiple parameters at the same time can help you compare data and look for individual signals that don’t match up to actual operating conditions. Although scan tool graphing isn’t equivalent in quality and accuracy to a lab scope reading, it can provide a comparative analysis of the activity in the two, three, four or six oxygen sensors found in most OBD II systems. Many scan tools are capable of storing a multiple-frame movie of selected PIDs. The scan tool can be programmed to record a movie after a specific DTC is stored in the PCM. Alternatively, the scan tool movie might be triggered manually when a driveability symptom occurs. In either case, you can observe the data or download it and print it later. Several software programs let you download a movie, then plot the values in a graphical display on your computer monitor.
Make the Most of What You’ve Got Take the time to learn what your scan tool will do when connected to a specific make or model. Do your best to gather all relevant information about the vehicle system being tested. That way you can get the most out of what the scan tool and PCM have to offer. The OBD II system won’t store a DTC unless it sees (or thinks it sees) a problem that can result in increased emissions. The only way to know what the PCM sees (or thinks it sees) is to look through the window provided by the scan tool interface. You have a DTC and its definition. You have freeze frame data that may help you zero in on the affected component or subsystem. PIDs have already provided you with additional clues about the operation of critical sensors. Keep your diagnosis simple as long as you can. Now fix the car. Visit www.motor.com to download a free copy of this article.
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Photo & screen captures: Bob Pattengale
INTERPRETING GENERIC SCAN DATA BY BOB PATTENGALE
Readily available ‘generic’ scan data provides an excellent foundation for OBD II diagnostics. Recent enhancements have increased the value of this information when servicing newer vehicles.
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f you don’t have a good starting point, driveability diagnostics can be a frustrating experience. One of the best places to start is with a scan tool. The question asked by many is, “Which scan tool should I use?” In a perfect world with unlimited resources, the first choice would probably be the factory scan tool.
Unfortunately, most technicians don’t have extra-deep pockets. That’s why my first choice is an OBD II generic scan tool. I’ve found that approximately 80% of the driveability problems I diagnose can be narrowed down or solved using nothing more than OBD II generic parameters. And all of that information is available on an OBD II generic scan tool that can be purchased for under $300. The good news is the recent phase-in
of new parameters will make OBD II generic data even more valuable. Fig. 1 on page 54 was taken from a 2002 Nissan Maxima and shows the typical parameters available on most OBD IIequipped vehicles. As many as 36 parameters were available under the original OBD II specification. Most vehicles from that era will support 13 to 20 parameters. The California Air Resources Board (CARB) revisions to OBD II CAN-equipped vehicles will increase
the number of potential generic parameters to more than 100. Fig. 2 on page 56 shows data from a CAN-equipped 2005 Dodge Durango. As you can see, the quality and quantity of data has increased significantly. This article will identify the parameters that provide the greatest amount of useful information and take a look at the new parameters that are being phased in. No matter what the driveability issue happens to be, the first parame-
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INTERPRETING GENERIC SCAN DATA
Fig. 1
ters to check are short-term fuel trim (STFT) and long-term fuel trim (LTFT). Fuel trim is a key diagnostic parameter and your window into what the computer is doing to control fuel delivery and how the adaptive strategy is operating. STFT and LTFT are expressed as a percentage, with the ideal range being within 5%. Positive fuel trim percentages indicate that the powertrain control module (PCM) is attempting to enrichen the fuel mixture to compensate for a perceived lean condition. Negative fuel trim percentages indicate that the PCM is attempting to enlean the fuel mixture to compensate for a perceived rich condition. STFT will normally sweep rapidly between enrichment and enleanment, while LTFT will remain more stable. If STFT or LTFT exceeds 10%, this should alert you to a potential problem. The next step is to determine if the condition exists in more than one op-
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erating range. Fuel trim should be checked at idle, at 1500 rpm and at 2500 rpm. For example, if LTFT B1 is 25% at idle but corrects to 4% at both 1500 and 2500 rpm, your diagnosis should focus on factors that can cause a lean condition at idle, such as a vacuum leak. If the condition exists in all rpm ranges, the cause is more likely to be fuel supply-related, such as a bad fuel pump, restricted injectors, etc. Fuel trim can also be used to identify which bank of cylinders is causing a problem. This will work only on bankto-bank fuel control engines. For example, if LTFT B1 is 20% and LTFT B2 is 3%, the source of the problem is associated with B1 cylinders only, and your diagnosis should focus on factors related to B1 cylinders only. The following parameters could affect fuel trim or provide additional diagnostic information. Also, even if fuel trim is not a concern, you might find an indication of another problem
when reviewing these parameters: Fuel System 1 Status and Fuel System 2 Status should be in closedloop (CL). If the PCM is not able to achieve CL, the fuel trim data may not be accurate. Engine Coolant Te m p e r a t u re (ECT) should reach operating temperature, preferably 190°F or higher. If the ECT is too low, the PCM may richen the fuel mixture to compensate for a (perceived) cold engine condition. Intake Air Temperature (IAT) should read ambient temperature or close to underhood temperature, depending on the location of the sensor. In the case of a cold engine check— Key On Engine Off (KOEO)—the ECT and IAT should be within 5°F of each other. The Mass Airflow (MAF) Sensor, if the system includes one, measures the amount of air flowing into the engine. The PCM uses this information to calculate the amount of fuel that
INTERPRETING GENERIC SCAN DATA
Fig. 2
should be delivered, to achieve the desired air/fuel mixture. The MAF sensor should be checked for accuracy in various rpm ranges, including wide-open throttle (WOT), and compared with the manufacturer’s recommendations. Mark Warren’s Dec. 2003 Driveability Corner column covered volumetric efficiency, which should help you with MAF diagnostics. A copy of that article is available at www.motor.com, and an updated volumetric efficiency chart is available at www.pwrtraining.com. When checking MAF sensor readings, be sure to identify the unit of measurement. The scan tool may report the information in grams per second (gm/S) or pounds per minute (lb/min). For example, if the MAF sensor specification is 4 to 6 gm/S and your scan tool is reporting .6 lb/min, change from English units to metric units to obtain accurate readings. Some technicians replace the sensor, only to realize later that the scan tool was not set correctly. The scan tool manufacturer might display the para-
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meter in both gm/S and lb/min to help avoid this confusion. The Manifold Absolute Pressure (MAP) Sensor, if available, measures manifold pressure, which is used by the PCM to calculate engine load. The reading in English units is normally displayed in inches of mercury (in./Hg). Don’t confuse the MAP sensor parameter with intake manifold vacuum; they’re not the same. A simple formula to use is: barometric pressure (BARO) MAP intake manifold vacuum. For example, BARO 27.5 in./Hg MAP 10.5 intake manifold vacuum of 17.0 in./Hg. Some vehicles are equipped with only a MAF sensor, some have only a MAP sensor and some are equipped with both sensors. Oxygen Sensor Output Voltage B1S1, B2S1, B1S2, etc., are used by the PCM to control fuel mixture. Another use for the oxygen sensors is to detect catalytic converter degradation. The scan tool can be used to check basic sensor operation. Another way to test oxygen sensors is with a graphing
scan tool, but you can still use the data grid if graphing is not available on your scanner. Most scan tools on the market now have some form of graphing capability. The process for testing the sensors is simple: The sensor needs to exceed .8 volt and drop below .2 volt, and the transition from low to high and high to low should be quick. In most cases, a good snap throttle test will verify the sensor’s ability to achieve the .8 and .2 voltage limits. If this method does not work, use a bottle of propane to manually richen the fuel mixture to check the oxygen sensor’s maximum output. To check the low oxygen sensor range, simply create a lean condition and check the voltage. Checking oxygen sensor speed is where a graphing scan tool helps. Fig. 3 on page 57 and Fig. 4 on page 58 show examples of oxygen sensor data graphed, along with STFT, LTFT and rpm, taken from two different graphing scan tools. Remember, your scan tool is not a lab scope. You’re not measuring the
Fig. 3
sensor in real time. The PCM receives the data from the oxygen sensor, processes it, then reports it to the scan tool. Also, a fundamental OBD II generic limitation is the speed at which that data is delivered to the scan tool. In most cases, the fastest possible data rate is approximately 10 times a second with only one parameter selected. If you’re requesting and/or displaying 10 parameters, this slows the data sample rate, and each parameter is reported to the scan tool just once per second. You can achieve the best results by graphing or displaying data from each oxygen sensor separately. If the transition seems slow, the sensor should be tested with a lab scope to verify the diagnosis before you replace it. Engine Speed (RPM) and Ignition Timing Advance can be used to verify good idle control strategy. Again, these are best checked using a graphing scan tool. The RPM, Vehicle Speed Sensor (VSS) and Throttle Position Sensor (TPS) should be checked for accuracy.
These parameters can also be used as reference points to duplicate symptoms and locate problems in recordings. Calculated Load, MIL Status, Fuel Pressure and Auxiliary Input Status (PTO) should also be considered, if they are reported.
Additional OBD II Parameters Now, let’s take a look at the more recently introduced OBD II parameters. These parameters were added on 2004 CAN-equipped vehicles, but may also be found on earlier models or nonCAN-equipped vehicles. For example, the air/fuel sensor parameters were available on earlier Toyota OBD II vehicles. Fig. 2 was taken from a 2005 Dodge Durango and shows many of the new parameters. Parameter descriptions from Fig. 2 are followed by the general OBD II description: FUEL STAT 1 Fuel System 1 Status: Fuel system status will display more than just Closed Loop (CL) or Open Loop (OL). You might find one
of the following messages: OL-Drive, indicating an open-loop condition during power enrichment or deceleration enleanment; OL-Fault, indicating the PCM is commanding open-loop due to a system fault; CL-Fault, indicating the PCM may be using a different fuel control strategy due to an oxygen sensor fault. ENG RUN TIME Time Since Engine Start: This parameter may be useful in determining when a particular problem occurs during an engine run cycle. DIST MIL ON Distance Traveled While MIL Is Activated: This parameter can be very useful in determining how long the customer has allowed a problem to exist. COMMAND EGR EGR_PCT: Commanded EGR is displayed as a percentage and is normalized for all EGR systems. EGR commanded OFF or Closed will display 0%, and EGR commanded to the fully open
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INTERPRETING GENERIC SCAN DATA
Fig. 4
position will display 100%. Keep in mind this parameter does not reflect the quantity of EGR flow—only what the PCM is commanding. EGR ERROR EGR_ERR: This parameter is displayed in percentage and represents EGR position errors. The EGR Error is also normalized for all types of EGR systems. The reading is based on a simple formula: (Actual EGR Position Commanded EGR) Commanded EGR EGR Error. For example, if the EGR valve is commanded open 10% and the EGR valve moves only 5% (5% 10%) 10% 50% error. If the scan tool displays EGR Error at 99.2% and the EGR is commanded OFF, this indicates that the PCM is receiving information that the EGR valve position is greater than 0%. This may be due to an EGR valve that is stuck partially open or a malfunctioning EGR position sensor. EVAP PURGE EVAP_PCT: This parameter is displayed as a percentage and is normalized for all types of purge systems. EVAP Purge Control
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commanded OFF will display 0% and EVAP Purge Control commanded fully open will display 100%. This is an important parameter to check if the vehicle is having fuel trim problems. Fuel trim readings may be abnormal, due to normal purge operation. To eliminate EVAP Purge as a potential contributor to a fuel trim problem, block the purge valve inlet to the intake manifold, then recheck fuel trim. FUEL LEVEL FUEL_PCT: Fuel level input is a very useful parameter when you’re attempting to complete system monitors and diagnose specific problems. For example, the misfire monitor on a 1999 Ford F-150 requires the fuel tank level to be greater than 15%. If you’re attempting to duplicate a misfire condition by monitoring misfire counts and the fuel level is under 15%, the misfire monitor may not run. This is also important for the evaporative emissions monitor, where many manufacturers require the fuel level to be above 15% and below 85%.
WARM-UPS WARM_UPS: This parameter will count the number of warm-ups since the DTCs were cleared. A warm-up is defined as the ECT rising at least 40°F from engine starting temperature, then reaching a minimum temperature of 160°F. This parameter will be useful in verifying warm-up cycles, if you’re attempting to duplicate a specific code that requires at least two warm-up cycles for completion. BARO BARO: This parameter is useful for diagnosing issues with MAP and MAF sensors. Check this parameter KOEO for accuracy related to your elevation. C AT TMP B1S1/B2S1 CATEMP11, 21, etc.: Catalyst temperature displays the substrate temperature for a specific catalyst. The temperature value may be obtained directly from a sensor or inferred using other sensor inputs. This parameter should have significant value when checking catalyst operation or looking at reasons for premature catalyst failure, say, due to overheating.
INTERPRETING GENERIC SCAN DATA
Fig. 5
CTRL MOD (V) VPWR: I was surprised this parameter was not included in the original OBD II specification. Voltage supply to the PCM is critical and is overlooked by many technicians. The voltage displayed should be close to the voltage present at the battery. This parameter can be used to look for low voltage supply issues. Keep in mind there are other voltage supplies to the PCM. The ignition voltage supply is a common source of driveability issues, but can still be checked only with an enhanced scan tool or by direct measurement. ABSOLUT LOAD LOAD_ABS: This parameter is the normalized value of air mass per intake stroke displayed as a percentage. Absolute load value ranges from 0% to approximately 95% for normally aspirated engines and 0% to 400% for boosted engines. The information is used to schedule spark and EGR rates, and to determine the pumping efficiency of the engine for diagnostic purposes. OL EQ RATIO EQ_RAT: Commanded equivalence ratio is used to determine the commanded air/fuel ratio of the engine. For conventional oxygen sensor vehicles, the scan tool should display 1.0 in closed-loop and the PCM-
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commanded EQ ratio during openloop. Wide-range and linear oxygen sensors will display the PCM-commanded EQ ratio in both open-loop and closed-loop. To calculate the actual A/F ratio being commanded, multiply the stoichiometric A/F ratio by the EQ ratio. For example, stoichiometric is a 14.64:1 ratio for gasoline. If the commanded EQ ratio is .95, the commanded A/F is 14.64 0.95 13.9 A/F. TP-B ABS, APP-D, APP-E, COMMAND TAC: These parameters relate to the throttle-by-wire system on the 2005 Dodge Durango of Fig. 2 and will be useful for diagnosing issues with this system. There are other throttle-by-wire generic parameters available for different types of systems on other vehicles. There are other parameters of interest, but they’re not displayed or available on this vehicle. Misfire data will be available for individual cylinders, similar to the information displayed on a GM enhanced scan tool. Also, if available, wide-range and linear air/fuel sensors are reported per sensor in voltage or milliamp (mA) measurements. Fig. 5 above shows a screen capture from the Vetronix MTS 3100 Mastertech. The red circle highlights the “greater than” symbol (>), indicating that multiple ECU responses differ in
value for this parameter. The blue circle highlights the equal sign (=), indicating that more than one ECU supports this parameter and similar values have been received for this parameter. Another possible symbol is the exclamation point (!), indicating that no responses have been received for this parameter, although it should be supported. This information will be useful in diagnosing problems with data on the CAN bus. As you can see, OBD II generic data has come a long way, and the data can be very useful in the diagnostic process. The important thing is to take time to check each parameter and determine how they relate to one another. If you haven’t already purchased an OBD II generic scan tool, look for one that can graph and record, if possible. The benefits will immediately pay off. The new parameters will take some time to sort out, but the diagnostic value will be significant. Keep in mind that the OBD II generic specification is not always followed to the letter, so it’s important to check the vehicle service information for variations and specifications. Visit www.motor.com to download a free copy of this article.
DATASTREAM
Photoillustration: Harold Perry; photos: Wieck Media & Jupiter Images
IN-DEPTH ANALYSIS BY SAM BELL We began this two-part article with a discussion of preliminary OBD II datastream analysis, conducted with the engine off. We’re going deeper this time, to explain the value of datastream information collected with the engine running.
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ast month’s installment on datastream analysis focused on the value of freeze frame data, Mode 5 and Mode 6 data and KOEO (key on, engine off) datastream. This month’s discussion picks up where we left off, with KOER (key on, engine running) analysis. So go ahead, start the engine! I recommend that KOER data collection always start in the generic, or global OBD II interface. Why? Because generic datastream PID values are never substitutes for actual sensor readings. For example, you can disconnect the MAP sensor connector on a Chrysler
product and drive it around while monitoring datastream in the enhanced (manufacturer-specific) interface. (Try this yourself; don’t just take my word for it.) You’ll see the MAP PID change along with the TPS sensor reading and rpm, showing a range of values that reflect likely MAP readings for each condition, moment by moment. These are substituted values. If you looked at the MAP voltage PID, however, it would show an unchanging reference voltage. In the enhanced interface, substitutions can and do occur. But in the generic interface, substituted values are never allowed. You would see MAP shown at a constant pressure equal to something a
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Chart & screen capture: Sam Bell
DATASTREAM IN-DEPTH ANALYSIS
Data collection and analysis might yield some helpful information, if you can find the wheat within the chaff. This is only a small portion of a larger data set with 100 values per PID.
bit higher than BARO. The generic interface allows calculated values, but never substituted values. So, what are we looking for, now that we’ve finally started the engine? The specific answer, of course, will depend largely on the details of the customer complaint and/or DTC(s) that are stored. We might, for example, be focusing on fuel trim numbers (and trends) if our code suggests an underlying air/fuel metering problem. We might be looking most closely at engine coolant temperature, and time-until-warm measurements when that seems warranted. Perhaps our problem lies in the evap area, or involves EGR flow. But ultimately, it doesn’t matter what the specific issue is; we’ll have to focus in on the systemic interactions that determine the overall characteristics of a particular data set. Here’s a concrete example to illustrate what I mean. The vehicle in question is a 1999 Chevy Venture minivan with the 3.4L V6. There was a DTC
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P0171 (Exhaust Too Lean, Bank 1) in memory with an active MIL. The sum of Short Term and Long Term Fuel Trims in freeze frame was in excess of
When evaluating a fuel trim trouble code, one of the first steps must always be to verify that the oxygen sensor (on which the DTC is based) is functioning correctly.
50%. Fuel pressure and volume had been verified as within specification. When evaluating a fuel trim trouble code, one of the first steps must always be to verify that the oxygen sensor (on which the DTC is based) is functioning correctly. During the test drive, I observed the O2 sensor switching rich, but not as often as would be expected if the very large fuel trim corrections shown were actually effective. Indeed, on the face of it, datastream seemed to confirm the DTC. Longtime readers, however, can probably anticipate what my next tests were: I checked the actual lambda value of the exhaust gases. Then I looked for a dynamic response as I artificially enriched the system with a blast of propane, then enleaned it by disconnecting a major vacuum hose. (See “What Goes In…Harnessing Lambda as a Diagnostic Tool” in the September 2005 issue of MOTOR. Search the index at www.motormagazine.com for all MOTOR magazine articles mentioned.) Hav-
Graphical representations of scan data “movies” can speed analysis. As an added bonus, using your scanner’s flight recorder mode allows you to concentrate on your driving. The data set here clearly points to a lack of adequate fuel volume. This graphical representation is derived from the exact same movie capture seen in the chart on the previous page.
ing found the idle lambda at a ridiculously low value of .85 (indicating a mixture with 15% more fuel than needed), I was not surprised to see that the O2 sensor didn’t register a rich condition until the engine was very nearly flooded with propane. When I removed the purge hose, engine rpm climbed and the engine smoothed out, while lambda marched toward the stoichiometric ideal value of 1.00. Once the faulty O2 sensor was replaced, all aspects of driveability improved, and the minivan returned to its previous fuel consumption levels. Dynamic tests verify DTC accuracy. In some instances, we may be able to utilize bidirectional controls embedded within our scan tool packages to actuate various components. In other cases, we may need to improvise, using signal simulators, power probes, jumpers, propane or just good, old-fashioned test driving as required to initiate change within the system we’re working on. (I’m not saying that it will always be as
easy as it was with the Venture. You and I know there will be problems that don’t set DTCs, problems that do set DTCs that have no apparent connection to the
One of the most powerful features of most scan tools— the so-called flight recorder—seems to be one of the least used. But it’s an analytical tool of considerable value.
actual root fault and, of course, problems that set appropriate codes yet are still really hard to diagnose.)
Floodlights and Spotlights One of the most powerful features of most scan tools is, as nearly as I can tell, one of the least used. This is the so-called flight recorder, data logger or movie mode. By whatever name it’s known, this is an analytical tool of considerable value. Take a look at the portion of saved scan data portrayed in the chart on page 38. As you see, any value in that information is well hidden. This might be termed a “floodlight” view, showing too many values for too many parameters. But look at the “spotlight” view above, where I’ve selected and graphed a few of the same PID values. This was a vehicle where there was no DTC stored in memory. By including both upstream O2 sensors, I have provided myself a cross-check, as there is less likelihood of
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DATASTREAM IN-DEPTH ANALYSIS both being bad. Similarly, MAF and rpm track nicely with one another, again providing a good cross-check. The data values at the cursor (the vertical line at frame 2) are called out at the left side of each PID’s plot. The upstream O2 sensors are switching nicely at 2000 rpm (as shown at frame ⫺51), but the graphic interface reveals an obvious problem at
higher speeds as the O2 sensors flat-line lean. A new fuel pump restored the missing performance.
Slow Motion and High Speed Moviemakers speed up or slow down the action on the screen by shooting at different numbers of frames per sec-
ond. When film shot at 20 frames per second is played back at 60 frames per second, the action seems to be occurring at three times the speed. Just as a 56k dial-up modem is slower than a DSL Internet connection, scan data transfer rates also vary according to the interface used. Generic communication modes often travel at a crawl, es-
Monitors 1.01
M
ost MOTOR readers have at least a passing familiarity with the concept of OBD II monitor completion status. Even so, a brief refresher may be in order. OBD II monitors are simply formalized sets of self-tests all related to a particular system or component. Continuous monitors. With a few very rare exceptions (mostly for 1998 and earlier models), the so-called continuous monitors always show up as “complete,” “done” or “ready.” Take this status report with a grain of salt. Unplug the IAT sensor, start the engine and check that the “Comprehensive Component Monitor” readiness status shows complete. Is the MIL on? Are there any pending codes? How long would you have to let the vehicle idle before it will trip the MIL and show a P0113 (IAT Sensor Circuit Voltage High) DTC? As it turns out, depending on the specific make, model and powertrain package, there are several specific criteria that must be met before the code will set. In one instance, the PCM must detect a VSS signal of 35 mph or more and an ECT value of 140°F or more, the calculated IAT must be less than ⫺38°F and all of these conditions must be met for at least 180 seconds of continuous duration, during which no other engine DTCs are set—all while MAF is less than 12 grams per second. (This particular example, incidentally, is a two-trip code. Some other manufacturers may make this and other DTCs under the component monitor’s jurisdiction into one- or twotrip codes, sometimes with even more complicated entry criteria.) Continuous monitors include the comprehensive component monitor, the fuel monitor and the misfire monitor. Each monitor runs continuously when conditions are appropriate, but not during all actual driving. For example, the misfire monitor is often suspended during 4WD operation, since feedback through the axles over rough roads might cause uneven disruption of the CKP signals, which could
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otherwise be misidentified as misfires. Similarly, extremely low fuel tank levels may suspend both misfire and fuel system monitors to avoid setting a DTC for running out of gas. Noncontinuous monitors. As I pointed out last month, it’s important to note the readiness status of the other, noncontinuous monitors as well. These are the monitors whose status will change to “incomplete,” “not ready” or “not done” when the codes are cleared. If a vehicle arrives at your shop showing one or more incomplete monitors, it’s likely that someone has already cleared the codes before it got to you. (There are a few vehicles—for example, some 1996 Subarus—which may reset monitor status to incomplete at every key-off, or other vehicles which may have certain monitors which cannot be made to run to completion in normal driving, such as the evap monitor on some Toyota Paseos.) If a vehicle shows up with incomplete monitors, however, you should certainly document that fact on your work order and be sure to advise the customer that there’s a very real possibility that one or more other codes may recur after the current repair has been completed. For more on this subject, see my article “How Not to Get MILStoned” in the April 2004 issue of MOTOR. More importantly, for our present purposes, the existence of incomplete monitors means that you may not be getting the whole picture as to what ails the vehicle you’re looking at. Keep an open mind, remembering that there may be other, as yet unknown issues hidden behind that incomplete monitor, and try not to rush your diagnosis. As mentioned in last month’s installment, there may be some valuable data accessible via Mode 6 even if the monitor is not complete, but there is a very real possibility that Mode 6 data for any incomplete monitor may turn out to be unreliable. And, of course, don’t overlook any pending DTCs. Remember, these do not illuminate the MIL, so you must seek them out on your own.
DATASTREAM IN-DEPTH ANALYSIS pecially in comparison to CAN speeds. If you’re stuck with a generic interface, you can often accomplish more by looking at less. The key here is PID selection. Choose the smallest number of PIDs that will give you the information you
actually need. Three or four are usually sufficient. This is your version of the filmmaker’s high-speed action trick, as you get more updates per unit time the fewer PIDs you select. With several hundred possible PIDs from which to choose, it’s just too
Lights Out?
I
t seems like a no-brainer: When you’re done with all your diagnostic tests and you’ve made the necessary repairs, you should turn off the MIL, right? That’s what your customer probably expects, and as we all know, meeting customer expectations is an important part of running a successful business. But there are often times when you should leave the MIL on. If your area uses an OBD II “plug & play” emissions test, the regulations usually require that no more than one monitor can be incomplete as of the time of testing for model year 2001 and newer vehicles, with no more than two incomplete monitors for 1996 to 2000 models. In some areas, retest eligibility requires that the converter monitor must show “complete” before a retest is valid. If an emissions test or retest is looming in your customer’s future, you and he must work out the pros and cons of clearing the codes and resetting the monitors to “incomplete.” If you clear the codes, the monitors will reset as well. This will require that someone will have to drive a sufficient number of monitors to completion before a retest will be valid. If local weather conditions, for example, will prevent the monitors from running in a timely way, your customer might be better off if you leave the MIL on. Then your customer would have to drive only those portions of the drive trace needed to run the monitor under which the current DTC set. For example, if you’re in the frigid climes of an upper Midwestern winter and a customer’s vehicle failed an emissions test because of a faulty O2 sensor heater, you’ll both be ahead if you don’t clear the code, letting it expire naturally as the heater monitor runs successfully to completion on the next two trips. This will avoid the necessity of rerunning all the rest of the monitors. Of course, if the vehicle failed the evap monitor, you’ll be better off clearing the code, because prolonged subfreezing temperatures may make running that particular monitor successfully virtually impossible for weeks at a time.
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easy to miss an intermittent data glitch, or to drown in a sea of too much information (see “Live Data vs. ‘Live Data’” on page 44). Most MOTOR readers are familiar with the ways in which some of the major OEMs have organized data PIDs for display in their enhanced scan tool interfaces. Groupings such as Misfire, Driveability, Emissions, Accessories and the like are good examples of the types of data sets you may want to construct while analyzing different sorts of problems. Tracking down a nasty intermittent problem? Don’t hesitate to pare down the OEM groupings even further to speed data updates.
Code-Setting Criteria and Operating Conditions If we’re trying to resolve a MIL-on complaint, it’s critical that we first review both the exact code-setting criteria and the operating conditions as revealed in our previously recorded freeze frame data. We’ll need to drive in such a way as to complete a good “trip” so the affected monitors can run to completion. (For a more detailed discussion of OBD II trips and monitors, see “Monitors 1.01” on page 40.) If we fail to meet the conditions under which the self-test (monitor) will run, we cannot hope to make progress. Using the previously recorded freeze frame parameters gives us a good general idea of the operating conditions required. Merely duplicating speed, load, temperature and other basic characteristics may not be enough. This is why we need to review and understand the details of the code-setting criteria and the monitor’s self-test strategy. For example, some monitors cannot run until others have already reached completion. A typical example would be a catalytic converter monitor that is suspended until the oxygen sensor monitors have run and passed. Some trouble codes, or even pending codes, suspend multiple monitors. Other vehicle faults may then go undetected until all monitors can run again. A P0500 (VSS Malfunction) in a Corolla, for example, will effectively suspend even the misfire monitor.
DATASTREAM ANALYSIS The net result is that we may have to clear the current DTCs and extinguish the MIL before our test drive can bear fruit. (But again, please be sure to read and record all the freeze frame data, the status of all monitors, the list of both current and pending DTCs and any available Mode 6 data before clearing the MIL (see “Lights Out?” on page 42). We’ll need to drive long enough to let the monitors in question reach completion. In some cases, this may require an extended period of time. Many Ford products, for example, normally require a minimum of a six-hour cold-soak before the evap monitor can run, although there may be ways to force this issue in some instances. Many Chrysler oxygen sensor monitors run only after engine shut-down (with key off),
Live Data vs. ‘Live Data’
I
ntermittent interruptions of sensor data can cause tricky driveability problems. Some glitches may set a DTC while others may not. While viewing datastream may reveal an intermittent sensor problem, it should not be relied upon to do so. The issue, once again, is in the data rate. Even a moderately fast interface, say the 41.6 kbps (kilobytes per second) J-1850 PWM used on many Ford products, can easily miss a several-millisecond dropout if it’s not that particular PID’s turn in the datastream. Where symptoms or DTCs point toward an intermittent sensor glitch, you’re probably better off breaking out your scope or graphing multimeter.
so that no amount of driving will ever bring them to completion. Certain monitors, and apparently even certain scan tools, may require a key-off sequence before the monitor status will update from incomplete to complete. MOTOR offers an excellent resource to help you understand these details—the OBD II Drive Cycle CD Version 7.0, available from your local MOTOR Distributor (1-800-4A-MOTOR). In some cases, local weather conditions may make monitor completion seem impossible until a later date, usually because of ambient temperature requirements, although sometimes as a result of road conditions. In most cases, however, it will still be possible to complete the monitor by running the vehicle on a lift or dynamometer. This option may occasionally result in setting, say, an ABS code, but most monitors can be run to completion swiftly and successfully on a lift. This option may also offer a safer, faster alternative to actual driving, as trees and telephone poles are less likely to jump in front of a vehicle on a stationary lift. Circle #22
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Conclusions Proper in-depth datastream analysis can often light the way toward correct diagnosis of driveability concerns. Recording all available DTCs, pending DTCs, freeze frame data and Mode 5 and Mode 6 results before clearing any DTCs is essential. Specific setting criteria for each DTC are manufacturerdetermined, regardless of whether the code assigned is generic or manufacturer-specific. Freeze frame data sets can be used to recreate the operating conditions under which a previous failure occurred and can help illuminate the conditions under which certain self-tests are conducted. Mode 5 and Mode 6 test results can help in analyzing the type and extent of certain failures. KOEO datastream analysis can sometimes reveal sensor faults or rationality concerns that might otherwise be overlooked. Looking at KOEO and KOER datastream on a regular basis makes knowngood values familiar. Once you know
the correct values, the conditions accompanying problems identified by freeze frame are easier to spot. KOER data can highlight current problems, es-
When trying to resolve a MIL-on complaint, it’s critical to first review the exact code-setting criteria and the operating conditions as revealed in the freeze frame data.
pecially when used in conjunction with graphical scanner interfaces. Generic data PIDs cannot include substituted values, and so may point up faults easily overlooked in more enhanced interfaces. Careful selection of customgrouped PIDs can provide faster scanner update rates. Pick your tools wisely. To verify hard faults, monitor datastream as you run actuator tests. Look for any mismatch between the command sent to a component and its actual response. For intermittent problems, record and graph data. In tough cases, test circuits with your scope or meter to verify actual voltage for comparison to specs. Used properly, these techniques will help you arrive quickly and confidently at an accurate diagnosis of the root cause of most driveability complaints. This article can be found online at www.motormagazine.com.
Circle #23
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O
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SUCCESSFUL
MAF SENSOR Photoillustration by Harold Perry; photos courtesy Wells Manufacturing Corp.
nce in a while we may encounter a total failure of a MAF sensor, one that is, perhaps, short circuited or internally open. Much more common, however, are failure modes in which the MAF sensor has become unreliable, underreporting or overreporting the true airflow into the engine. Indeed, as we shall see, many MAF sensor failures actually result in both underreporting and overreporting! Before we get down to brass tacks, a brief review of the basics of MAF systems is in order. Fuel control systems for most modern gasoline engines are centered either on MAF or MAP (manifold absolute pressure). MAF systems, which, as their name suggests, measure the weight of incoming air and then meter the appropriate amount of fuel to ensure efficient combustion, are potentially more precise, although MAP systems, which calculate fuel requirements based on engine load, have historically demonstrated greater reliability. As you already know, combustion is most efficient when the ratio of air to fuel is approximately 14.7:1 by weight. Mass and weight are essentially synonymous in the presence of a sufficiently strong gravitational field such as the Earth’s. Thus, knowing the weight of the air entering the engine allows the engine controller to meter the exact amount of fuel required to achieve efficient combustion. The controller commands the fuel injectors to open for an amount of time calculated to be sufficient to allow the correct weight of fuel to enter the engine, providing that the fuel’s pressure is known. Fuel delivery is fine-tuned by applying fuel trim corrections derived from the closed-loop feedback of the oxygen sensor(s). If the entire system is working as designed, fuel trim corrections, expressed as a percentage deviation from the base fuel delivery programming, will be within 10% (either positive or negative) of the programmed quantity. In the absence of a MAF-specific diagnostic trouble code (DTC), what would first lead us to even suspect that a faulty MAF sensor might underlie a particular driveability problem? To function correctly, all of the air
DIAGNOSIS BY SAM BELL A broad range of seemingly unrelated or contradictory driveability complaints may arise from MAF sensor performance faults. Use this guide to navigate out of a diagnostic thicket or, better still, to avoid one entirely.
entering an engine’s combustion chambers must be “seen” by the MAF sensor. This means that any vacuum or air leak downstream of the sensor will result in insufficient fuel metering, causing a lean condition in open-loop operation and higher-than-normal fuel trim values in closed-loop. When we encounter a MAF sensor-equipped vehicle exhibiting these symptoms, we need
to check for unmetered airflow first. Remember, too, that unmetered airflow may not require an external air leak. An incorrectly applied or faulty PCV valve can result in incorrect MAF data where the PCV intake through the breather hose is upstream of the MAF. So, the first two rules of MAF sensor diagnosis are: 1. Find and eliminate all external air
or vacuum leaks downstream of the MAF sensor. When in doubt, use a smoke machine, or lightly pressurize the intake manifold and spray with a soap & water solution. 2. Verify that the manufacturer-specified PCV valve is correctly installed and functioning as designed. (This is one instance where precautionary replacement may be cost-justified.)
Only after these two steps have been completed can you safely proceed with other diagnostics. The foremost clue that the fault lies with the MAF sensor itself will be excessive fuel trim corrections, usually negative at idle, more or less normal in midrange operation and positive under high load conditions (see “How Contamination Affects Hot-Wire & Hot-Film MAF Sensors” on page 32).
While there are several distinct MAF sensor technologies ranging from hotwire or hot-film to Karman vortex and Corialis sensors, and while MAF sensor outputs may take the form of variable frequency, variable current or a simple analog voltage, the diagnostic principles remain largely the same. Let’s start with Ford vehicles, for a couple of reasons. First, they are so widespread that most of us are familiar with them. Second, most MAF sensorequipped Ford products make use of a PID (Parameter IDentification) called BARO (barometric pressure). Up to 2001 models, this was an inferred, or calculated, value generated by the PCM (powertrain control module) in response to the maximum MAF flow rates observed on hard wide-open throttle (WOT) acceleration. Where this calculated BARO PID is available, it is of great diagnostic value, since it can confirm MAF sensor accuracy, if only under high flow rate conditions. To use the BARO PID, you must first know your approximate local barometric pressure. You might consult the BARO PID on a known-good MAP sensor-equipped vehicle. Alternatively, your local airport can provide this data. Do not rely on local weather stations, however, since these usually report a “corrected” barometric pressure. If weather information is the only available source, a rule of thumb is to subtract about 1 in. of mercury (1 in./Hg) for every 1000 ft. of elevation above sea level. This will yield a rough estimate of your actual local barometric pressure. For greater accuracy, you can purchase a functional barometer for something less than $40. Compare this data with the BARO PID. A large discrepancy here—say, more than 2 in./Hg—should direct your suspicions toward the MAF. Confirm your hypothesis as follows: First, make sure you have followed the steps outlined in the two rules above. Next, record all freeze frame data and all DTCs, including pending DTCs. If the OBD monitor readiness status for oxygen sensors shows READY, proceed to the next step. If it doesn’t, refer to the procedures in the following paragraph now. Next, perform a KAM (Keep Alive Memory) reset and drive the vehicle. Make sure your test drive
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Fig. 1 includes at least three sustained WOT accelerations. (It’s not necessary to speed to accomplish a sustained WOT acceleration. Rather than a WOT snap from idle, an uphill downshift at 20 to 30 mph is usually sufficient. The WOT prescription can be met at throttle openings as low as 50% to 70%.) The BARO PID should update from its default reading by the end of the third WOT acceleration. If it’s now close to your local barometric pressure, the MAF sensor is not likely to be faulty. If BARO is not close, try one of the cleaning techniques explained in the sidebar “Keeping It Clean” on page 34, then again reset KAM and take a test drive. If the BARO is still out of range, a replacement MAF sensor is in your customer’s future. Unfortunately, in many 2002 and later Fords, the calculated BARO PID is supplanted by a direct BARO reading
Fig. 3
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Fig. 2 taken from a sensor incorporated into the ESM (EGR System Management) valve, greatly lessening its diagnostic value for our current purposes. If the oxygen sensor monitor status showed INCOMPLETE above, you’ll have to verify O2 sensor accuracy and performance before performing the KAM reset procedure. Use a 4- or 5-gas analyzer to determine whether the air/fuel ratio is correct in closed-loop operation. The notes about lambda () below should help. Outside of the Ford family, MAF sensor diagnosis is more difficult. Large fuel trim corrections—either positive or negative—are often the only initial pointer to MAF sensor problems. Again, any and all air leaks downstream of the MAF sensor must be repaired first. Since accurate fuel trim corrections depend on correct O2 sensor out-
puts, you must verify the functionality of these sensors first. The easiest and fastest way to do this is by checking lambda, a type of measure of the air/fuel ratio. (For a detailed explanation, see my article in the September 2005 issue of MOTOR.) If the O2 sensors are functioning correctly, lambda at idle should be very nearly equal to 1.00 in closedloop. You may wish to check this also at 1500 to 1800 rpm to verify adequate mixture control off idle. Once lambda is found to be correct, the O2 sensors are proven good. Then any fuel trim adjustments must result from unmetered or incorrectly metered airflow or from incorrect fuel delivery. Distinguishing between fuel delivery problems and MAF sensor problems can be very frustrating. Start by verifying fuel pressure and volume. (Those who rely on pressure alone may regret
Fig. 4
Screen captures: Sam Bell
SUCCESSFUL MAF SENSOR DIAGNOSIS
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 5 it.) Use your scan tool to record critical data PIDs and graph them for analysis. Here are a couple of examples: In Fig. 1 on page 30, taken during a period of closed-loop operation, shortterm fuel trims (blue and green traces) for each bank were above 13% at 1100 rpm (red trace), yet dropped sharply negative at 3600 rpm, proving that inadequate fuel delivery was not the problem. The values indicated in the legend boxes correspond to the readings obtained
Fig. 6 at the indicated cursor position (vertical black line). The vertical white line indicates the trigger point for the recording. Subsequent diagnostics focused on the MAF sensor and the PCV system. Take a look at the scan data graph shown in Fig. 2. It shows a car whose faulty fuel pump was unable to deliver sufficient fuel under high load conditions. Notice the very low O2 sensor readings (displayed in blue) corresponding to the cursor (black vertical
How Contamination Affects Hot-Wire & Hot-Film MAF Sensors
H
ot-wire and hot-film MAF sensors calculate airflow based on monitoring the current required to maintain a constant temperature in the sensing element. When dirt accumulates, the additional surface area allows greater heat dissipation at low airflow rates. The dirt, however, also functions as an insulator, with an overall net resistance to heat transfer at very high airflow rates. At idle and under relatively low flow/load rate conditions where the majority of operation may take place, the surface area effect usually predominates, causing a rich condition with fuel trim corrections usually in the range of ⫺10% to ⫺5%. At sustained high flow/load rates, the insulative effect usually takes over, causing a lean mixture needing fuel trim corrections as high as +30%. Worse still is a complex case of
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“mass confusion” that may arise under hard acceleration when longterm negative fuel trim corrections, learned in closed-loop under lowflow-rate conditions, are applied precisely when positive fuel trim corrections would be more appropriate. So, for example, when the system goes to open-loop during hard acceleration where the MAF is already underreporting airflow by up to 30%, the PCM may subtract an additional 10% to 15% (LTFT) from the normal fuel delivery calculation, leaving the system as much as 45% leaner than desired! In midrange operation, the two effects (surface area and insulative properties) may roughly cancel each out, with fuel trims being more or less normal. Additionally, the exact chemistry and configuration of dirt buildups can vary, changing the balance of power between the surface area and insulative effects.
line just to the right of the zero time stamp). Fuel pressure was within spec at idle and at about 2000 rpm, but volume was very low. The sudden dropoff in O2 activity in response to hard acceleration is a characteristic observed in many instances of MAF sensor faults as well. Ultimately, known-good snapshots, waveforms and other data sets are invaluable. Take a look at the scan snapshot in Fig 3. Does it show good fuel trim and appropriate MAF sensor readings? Since total fuel trim stays well within the 0 ±10% range throughout the trace, it’s a good bet that the MAF sensor is working well, at least under the sampled conditions. How about the data set shown in Fig. 4? In fact, the snapshot was taken during open-loop, closed-throttle deceleration when fuel was not being injected, so the O2 sensor PID makes sense. It’s actually a substituted default value inserted whenever the vehicle is in closed-throttle decel mode. What about the reported MAP value? A reading of 4.00 in./Hg shows very high engine vacuum, which jibes with the reported TPS PID. The fuel trim data is within the usually accepted range of 0 ±10%. Good data can come in a variety of formats. Of course, waveform captures from your scope are often all that are needed to confirm a faulty MAF sensor. In our shop, we’ve found that a snap-throttle MAF test for Ford products should always produce a peak voltage of at least 3.8 volts DC. The snap-throttle test is
SUCCESSFUL MAF SENSOR DIAGNOSIS performed the same way as for ignition analysis. The idea is not to race the engine, but simply to open the throttle abruptly to allow a momentary surge of maximum airflow as the intake manifold gets suddenly filled with air. It’s critical that the throttle be opened (and closed) as quickly as possible during this test. The waveform in Fig. 5 on page 32 is from a known-good MAF sensor. Note the peak voltage of 3.8 volts. The rapid rise and fall after the throttle was first opened is normal and reflects the initial gulp of air hitting the intake manifold walls and suddenly reaching maximum density, greatly reducing subsequent flow. The exact shape of the waveform may vary from model to model, based on intake manifold and air duct (snorkel) design. What’s the relationship between MAF and engine speed? As Fig. 6 shows, rpm and airflow rate track one another closely under the moderate acceleration conditions during which this screen capture was taken. The similarity of the shapes of the two traces shown
Fig. 7 here suggests, but does not prove, that the MAF sensor is functioning well under these conditions. If the airflow report was consistently increased or decreased by the same factor, say 10% or even 50%, the shape of its graph would remain the same. Consider the additional plots presented in Fig. 7 above. Does the extra data shed any light on the MAF sensor’s accuracy? Or is this just an example of too much information? Since short-term and long-term fuel
Keeping It Clean
M
ost MAF sensor failures result from contamination. Sometimes the dirt is visible, but more often it’s not. Technicians have tried a variety of cleaners, with mixed success. Many use an aerosol brake/electrical parts cleaner, waiting until the MAF sensor is cold. A Ford trainer in my area swears by the most popular consumer glass cleaner. Several top technicians report good results from steam cleaning, while others prefer a spray induction cleaner. The vast majority of technicians warn that the MAF sensor may be damaged by any type of cleaning where the electrical connector is not held upright. This is particularly true where strong chemicals are used, as they may pool and work their way
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into the delicate electronic circuitry. To avoid future contamination, be wary of oiled air filters or any that appear likely to shed lint. Poor sealing of air filter housings may contribute to contamination. Never spray an ill-fitting air filter with a silicone lubricant or sealer; such sprays are likely to render the MAF sensor inaccurate. If an engine produces excessive blowby gases, these may contaminate the MAF sensor, as well. Be sure any specified filter breather element is installed. If none is specified, but oil accumulates in the air intake housing, the MAF sensor or associated intake ducts, be sure to investigate and remedy the cause to prevent repeat failures. Be sure to check manufacturers’ TSBs, the iATN archives and other sources as well.
trims remain within single digits throughout, we can be reasonably sure that the MAF sensor is functioning correctly. Do we really benefit from looking at the O2 sensor data here? We could probably do almost as well without it, since we have both STFT and LTFT, but the O2 trace (blue) serves as an additional crosscheck on the validity of the fuel trim calculations. More importantly, the O 2 sensor trace proves both that an appropriately rich mixture was obtained on hard acceleration and that applied fuel trim corrections were effective throughout the captured data set. I said at the outset that hard failures were relatively rare, but they do occur from time to time, and I owe it to you to discuss this type of failure as well as intermittent failures. Open-circuited or short-circuited MAF sensors usually set a trouble code, most frequently P0102 or P0103 (low input and high input, respectively). P0100 is a nonspecific MAF sensor circuit fault, while P0104 indicates an intermittent circuit failure. Checking scan data is a vital first step toward successful diagnosis of any of these codes. On pre-OBD II vehicles especially, unplugging a faulty MAF sensor will often restore a minimum degree of driveability as the PCM reverts to TPS, rpm and/or MAP as fuel determinants. Certain mid-’80s GM vehicles were notorious for intermittent MAF sensor failures. These usually could be easily recreated by lightly tapping with a small screwdriver on the MAF sensor housing at idle. A noticeable stumble occurring with each tap clinches the condemnation (Fig. 8, page 36). Of course, backprobing the MAF sensor connector for voltage drops at both the power and ground terminals KOER is a required step before any final condemnation. The coincidence of VBATT and MAF both showing 0.0 volts cannot be ignored. Neither should the mouse nest in the MAF, nor the gnawed wires throughout the engine compartment. Why is this a hard diagnosis? Conta-
SUCCESSFUL MAF SENSOR DIAGNOSIS minated MAF sensors often overreport airflow at idle (resulting in a rich condition and negative fuel trim corrections) while underreporting airflow under load (resulting in a lean condition and positive fuel trim corrections). This double whammy makes diagnosis more difficult for a number of reasons: First, many technicians incorrectly eliminate the MAF sensor as a potential culprit because they expect it to show the same bias (either over- or underreporting) throughout its operating range. Second, a lack of a direct MAF fault DTC (such as P0100) is often mistaken to mean that the MAF sensor must be good. Third, the symptoms mimic (among other possibilities) those of a vehicle suffering from low fuel pump output coupled with slightly leaking injectors or an overly active canister purge system. Even sluggish, contaminated or
Fig. 8 biased oxygen sensors may cause similar symptoms. Without appropriate testing, it’s hard to distinguish—just by driving—among certain ignition or knock sensor faults and MAF sensor malfunctions. Additionally, since MAF sensors are somewhat pricey, many technicians are afraid to condemn them, fearing either the customer’s or the boss’ wrath if their diagnosis is not borne out. Perhaps
the biggest obstacle is lack of a comprehensive database of known-good waveforms, voltages and scan data against which to compare the suspect. My own data set features known-good scan data and scope captures made KOEO, at idle and on snap-throttle. In general, these three data points should be sufficient to identify a faulty MAF sensor even before it sets a fuel trim code. A bad Bosch hot-wire MAF sensor may be the result of a failed burn-off circuit. Don’t simply replace the sensor; make sure the burn-off is functional. (The purpose of the burn-off is to clean the hot-wire of contaminants after each trip.) Burnoff is usually a key OFF function after engine operation exceeding 2000 rpm. Burn-off circuit faults may be in the PCM or a relay. The hot-wire should glow visibly red during burn-off. So what can we conclude from all this? A broad and seemingly unrelated or even contradictory range of fuel system-related driveability complaints may arise from MAF sensor performance faults. Fuel trim data showing excessive corrections from base programming casts strong suspicion on MAF sensor performance issues. After recording all DTCs and freeze frame data, many experienced techs recommend unplugging a suspect MAF sensor to see if basic driveability is improved. Scope traces at idle and on snap-throttle acceleration help verify MAF sensor guilt or innocence. As usual, a library of known-good scan data and waveforms is invaluable. The Min/Max voltage feature on your DMM may not be fast enough to catch actual peak voltage on a snap-throttle test, but is usually sufficient for verifying performance of frequency-generating (digital) MAF sensors. If your scope is capable of pulse-width triggering, using that function will provide exact captures of digital MAF sensors in snapthrottle testing. Visit www.motor.com to download a free copy of this article.
Circle # 27
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DOING IT ALL WITH
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Photoillustration: Harold A. Perry; images: Thinkstock, David Kimble, Sun & General Motors
T
he Greek root gen- under lies many words in common parlance—generic is one of them. Your medical insurance provider and your pharmacist both know that when it comes to prescriptions, generic equivalents can save us all big money, with identical results. In the last few years, generic has, for complex reasons, become a pejorative term, often used to convey the idea of something of lesser quality than a socalled name-brand alternative. Yet, for diagnostic purposes, the generic datastream sometimes offers a better window into powertrain management operating conditions than even the name-brand “enhanced” or “manufacturer-specific” interface can. In fact, even though I own several much more powerful and expensive scan tools, I routinely use the generic interface residing on the cheapest of the bunch as my go-to choice for initial code retrieval and data analysis. This particular machine, an aging AutoXray EZ6000, offers no bidirectional controls above code clearing, but has the signal virtues of speed and a very high overall connectivity rate. It also quickly compiles a printable report which includes current operating PIDs, DTCs (including pending codes) and freeze frame data, all of which are obviously useful. Individual monitor completion status requires a separate query, as do both Mode $05 (oxygen sensor test results) and Mode $06 (monitor self-test results) data. Its nice graphing program makes data analysis easy after a road test, and I’m actually happy that you cannot both read and record data simultaneously, as the trees in my neighborhood view that particular behavioral combination as an excuse to jump out in front of you. One of the primary benefits of the
BY SAM BELL A resourceful diagnostician knows complicated and expensive equipment isn’t always needed. Tools with a narrower focus, combined with an enlightened approach, allow him to get the job done.
GENERIC DATASTREAM generic datastream stems from the requirement that it not display substituted values. While many so-called enhanced interfaces offer access to a greater number of data PIDs, some of these may reflect substituted values not based on current operating data. For example, most Chrysler products will substitute a reasonable guess for the actual intake manifold vacuum value when the MAP sensor is unplugged. If you look in the enhanced datastream, you’ll see that value varying quite believably as you rev the engine or drive the vehicle. If you look at MAP_Volts, however, you’ll see a fixed value reflecting reference voltage (Vref) for the sensor circuit. But how often do you actually look at that PID instead of the vacuum reading? While substituted values are prohibited in the generic datastream, calculated values are not. Thus, for example, an ECT PID of ⫺40°F reflects the calculated temperature of an open ECT sensor circuit. In such cases, Toyota, for example, has for many years, then substituted a value of 176°F in its enhanced datastream, but not in the generic data. In our unplugged Chrysler MAP sensor example above, using a generic interface, you’d see an unmoving value of something in the neighborhood of 255kPa or higher, corresponding to a boost pressure of about 25 psi above atmospheric. As a technical consultant to our state EPA, several times a year I encounter vehicles which have failed our OBD II plug & play state emissions test for a MIL-on condition with one or more current DTCs that simply do not appear in the “enhanced” interface, but which are readily retrieved using a generic hookup. I’m afraid I can’t shed a lot of light on why this would occur since, clearly, it should not. Thus far, I have not encountered this issue in any 2008 or later vehicles. There seem to be a few makes
July 2014
21
which are more prone to this problem, but my data set is too sparse to be certain of any meaningful correlation. For the moment, suffice it to say that the state’s testing interface is also a generic one, and, apparently, there are instances in which a DTC may set but not be retrieved via even the factory scan tool. On these occasions, only a generic interface will work. As the saying has it, truth is stranger than fiction. An additional advantage of using the generic datastream becomes apparent when you’re working on a vehicle for which your scan tool doesn’t provide an enhanced interface. Don’t laugh; I’ve had students call me up to ask what to do because they didn’t have a scan tool that offered, say, a Saab or Daihatsu option. A gentle reminder that they could at least start in the generic interface usually nets an embarrassed oops! Because the generic interface contains the data most critical to engine operations (see the starred items in the “Generic PIDs” list on page 24), it’s normally sufficient to rule in or rule out a particular area of concern such as fuel delivery, for example, early in the diagnostic process. While you might well prefer to work with a dealer-equivalent scan tool in almost all cases, in the real world you may not be able to justify buying a tool with limited utility vis-à-vis your regular customer base. Let’s take a look at what the generic interface typically offers these days (see the screen captures on this page). The J1979 SAE standard specifically defines 128 generic data PIDs, but not all manufacturers use or support all of them. Some, such as Mode $01, PID$6F (current turbocharger compressor inlet pressure), are highly specialized and won’t apply to most current-production vehicles, while others, such as PID $06 (engine RPM), are pretty universal. A typical PID list of current values (Mode $01) or of freeze frame values (Mode $02) would include some or all of the 74 items listed in the generic PIDs list. Your scan tool may use slightly different acronyms or abbreviations to identify various data items. Most of the PIDs in the list are probably familiar to you, but a few may have you scratching your head. As you see,
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Screen captures: Sam Bell
DOING IT ALL WITH GENERIC DATASTREAM
These two screen shots show the 60 lines of generic data available from a known-good 2014 Mazda CX-5, as captured via a Snap-on SOLUS Ultra scan tool.
starting with a model year 2005 phasein, several new parameters have been added to the original generic data list. These include both commanded and actual fuel-rail pressure, EGR command and EGR error calculation, commanded purge percentage, commanded equivalence ratio and a host of others, including many diesel-specific PIDs. In-use counters may also indicate how many times each of the various onboard monitors has run to completion since the codes were last cleared. The list on page 24 includes most of the generic PIDs currently in widespread use. However, since not all manufacturers support all PIDs, and since their choices may vary by model, engine and/or equipment, the list given here represents only a portion of the PIDs
potentially supported. Additionally, manufacturers are free to establish and define supplemental modes and PIDs which may or may not be accessible via a generic interface. All ECUs with authority or control over emissions-related issues, however, must be accessible via the generic interface. From our generic PIDs list, I want to focus on commanded equivalence ratios first. In essence, this is the PCM’s way of reporting how rich or lean a mixture it’s commanding. The PID is presented in a lambda format, with 1.0 indicating a stoichiometric (ideal) air/fuel ratio. Larger numbers indicate more air—a command to run at a leaner air/fuel ratio—while numbers less than 1.0 indicate a correspondingly richer mixture. If you have a gas analyzer capable of
DOING IT ALL WITH GENERIC DATASTREAM displaying lambda, it should coincide extremely well with the Commanded Equivalence PID. As with all fuel trim-related issues, it’s
T
he five starred (★) critical PIDs in the list below are the most influential inputs. Virtually all the others function merely to fine-tune (trim) the basic spark and fuel (base map) commands mapped out in response to these PIDs. The cause of any fuel trim corrections (STFT, LTFT) beyond the range of approximately ⫾5% must be investigated. Standards Compliance - such as OBD II (Federal), OBD II (CARB), EOBD (Europe), etc. MIL - malfunction indicator lamp status (off/on) MON_STAT - monitor completion status since codes cleared DTC_CNT - number of confirmed emissions-related DTCs available for display ★RPM - revolutions per minute: also, engine crankshaft (or eccentric shaft) speed, sourced from the CKP ★IAT - intake air temperature ★ECT - engine coolant temperature ★MAP and/or ★MAF - manifold absolute pressure or mass airflow, respectively ★TPS or ★TP - throttle position sensor, usually given as calculated percentage; see absolute TPS below CALC_LOAD - calculated, based on current airflow, as percentage of peak airflow at sea level at current rpm, with correction for current BARO LOOP - status: closed, closed with fault, open due to insufficient temperature, open due to high load or decel fuel cut, open due to system fault STFT_x (per bank) - shortterm fuel trim; the percentage of fuel added to or subtracted from the base fuel schedule (for speed, load, temperature, etc.) in order to achieve stoichiometry as determined by the relevant air/fuel or O2 sensor
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best to check this PID at idle, at about 1200 rpm and at about 2500 rpm. If your actual tailpipe measurements don’t coincide with the PID, be sure to check
for any exhaust leaks first. If there are none, you’ll have to check for factors that could account for the discrepancy, such as fuel pressure faults, vacuum
Generic PIDs LTFT_x (per bank) - long-term fuel trim VSS - vehicle speed sensor HO2SBxSy - heated oxygen sensor, Bank x, Sensor y, such as B1S2 for a bank 1 downstream sensor IGN_ADV - ignition timing, measured in crankshaft degrees SAS or SEC_AIR - commanded secondary air status off/on; may include information such as atmosphere, upstream or downstream of converter, commanded on for diagnostic purposes RUN_TIME - seconds since last engine start; some manufacturers stop the count at 255 seconds DISTANCE TRAVELED WITH MIL ON – in miles or km FRP - fuel rail pressure relative to intake manifold pressure FRP_G - fuel rail pressure, gauge reading O2Sx_WR_lambda(x) - wide range air/fuel sensor, bank x, equivalence ratio (0-1.999) or voltage (0-7.999) EGR - commanded EGR percentage EGR_ERR - deviation of sensed or calculated position from commanded position, percent PURGE - commanded percentage FUEL_LVL - fuel level input percentage; can provide especially invaluable information in freeze frame diagnostics of misfire codes set under “ran-out-ofgas” conditions; unfortunately, not universally implemented WARMUPS - number of warmups since codes cleared; a warm-up is an ECT increase of at least 40°F in which the ECT reaches at least 160°F DIST SINCE CLR - distance since codes cleared EVAP_PRESS - evaporative system pressure
BARO - absolute atmospheric pressure (varies with altitude and weather) O2Sx_WR_lambda(x) - equivalence ratio or current - wide range air/fuel sensor , position x, equivalence ratio (0-1.999) or current (-128mA to +127.99mA) CAT_TEMP BxSy - catalyst temperature by bank and position (may be wildly unreliable) MON_STAT - monitor status, current trip CONT_MOD_V - control module voltage; usually measured on the B+ input for the KeepAlive-Memory (KAM) but may be measured on a switched ignition input line ABS_LOAD - absolute load, percentage, 0-25,700% REL_TPS - relative throttle position percentage AMB_AT or AMB_TEMP - ambient air temperature; where used, usually measured in front of the radiator, while IAT or MAT (manifold air temperature) are usually collected in the intake ductwork, or inside the throttle body or intake manifold, respectively ABS_TPx - absolute throttle position, percentage, sensor B or C APP_x - accelerator pedal position sensors D-F TP_CMD - commanded throttle actuator percentage MIL_TIM - time run with MIL on, minutes FUEL_TYP - fuel type ETOH_PCT or ETH_PCT ethanol fuel % ABS_EVAP - absolute evap system vapor pressure, 0327.675kPa EVAP_P or EVAP_PRESS - evap system vapor pressure (gauge), from -32,767 to +32,768Pa STFTHO2BxS2 – short-term secondary (postcatalyst) oxygen sensor trim by bank
LTFTHO2BxS2 - long-term secondary oxygen sensor trim by bank HY_BATT_PCT - hybrid battery pack remaining life, percentage E_OIL_T or ENG_OIL_TEMP engine oil temperature INJ_TIM - fuel injection timing, in crankshaft degrees from ⫺210° BTDC to ⫹302° ATDC FUEL_RAT - engine fuel rate in volume per unit time—e.g., liters per hour, gallons per minute, etc. TRQ_DEM - driver’s demand engine, percent torque TRQ_PCT - actual engine, percent torque REF_TRQ - engine reference torque in Nm (0 to 65,535) TRQ_A-E - engine percent torque data at A=idle; B, C, D, E = defined points AFC - commanded diesel intake airflow control and relative intake airflow position EGR_TEMP - exhaust gas recirculation temperature COMP_IN_PRESS - turbocharger compressor inlet pressure BOOST - boost pressure control VGT – variable-geometry turbo control WAST_GAT - wastegate control EXH_PRESS - exhaust pressure TURB_RPM - turbocharger rpm TURB_TEMP - turbocharger temperature CACT - charge air cooler temperature EGTx - exhaust gas temperature, by bank DPF - diesel particulate filter DPF_T - diesel particulate filter temperature NOX - NOX sensor MAN_TEMP - manifold surface temperature NOX_RGNT - NOX reagent system PMS - particulate matter sensor
DOING IT ALL WITH GENERIC DATASTREAM leaks or a biased oxygen or air/fuel sensor. If you observe a close correlation with lambda, you’ll be able to use this PID with confidence in lieu of actual lambda readings while conducting additional tests. In general, you should expect this PID to read very close to 1.00 at idle in closed-loop operation with conventional oxygen sensors in the upstream positions. (Wide-range air/fuel [WRAF] ratio sensors may target alternate values under various driving conditions, typically targeting a leaner mix under lightthrottle cruise, for example. Additionally, vehicles using gasoline direct injection [GDI] may deviate from stoichiometry even at idle or under light-throttle cruise conditions.) Keep in mind that the name says a lot: This PID reports the command, not necessarily the effect of the command. Once in a blue moon you may find that commanded equivalence ratio seems to travel exactly opposite from lambda, so that a Com_Eq_Rat of .95 corresponds to an actual lambda value of 1.05, for instance. After the one instance in which I’ve encountered this, I eventually learned to think of the PID value as a deviation from 1.00, then move exactly that far in the opposite direction. (An unfortunate computer crash led to the OBD - on-board diagnostics. OBD II - second-generation OBD, as specified by SAE J1979. EOBD - Euro-specification OBD; slightly different from SAE-spec. JOBD - Japanese-specification OBD; slightly different from SAE-spec. DTC - diagnostic trouble code; Pcodes refer to powertrain management faults; U-codes flag communication network errors; B-codes relate to faults in body system management; C-codes are chassis system based. PDTC - Permanent DTC; one that cannot be cleared directly via scan tool command; such codes will selfclear after the affected monitors have successfully run to completion with no further faults. PDTCs are written into a section of nonvolatile memory, so they persist even if the
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Fuel Type Table: Mode $01, PID $51 Value Description 0 .........Not available 1 .........Gasoline 2 .........Methanol 3 .........Ethanol 4 .........Diesel 5...........LPG (liquid propane gas) 6...........CNG (compressed natural gas) 7 .........Propane 8 .........Electric 9..........Bifuel running gasoline 10..........Bifuel running methanol 11 ........Bifuel running ethanol 12 ........Bifuel running LPG 13 ........Bifuel running CNG 14.........Bifuel running propane 15 .........Bifuel running electricity 16.........Bifuel running electric and combustion engine 17 ........Hybrid gasoline 18 ........Hybrid ethanol 19 ........Hybrid diesel 20 ........Hybrid electric 21.........Hybrid running electric and combustion engine 22 ........Hybrid regenerative 23 ........Bifuel running diesel Any other value is reserved by ISO/SAE. There are currently no definitions for flexible-fuel vehicles.
Glossary battery is disconnected and all capacitors are discharged. PID - parameter identification; a value found in current or freeze frame data; may indicate a sensor reading, calculated value or command status. In a nongeneric (enhanced) interface, may indicate a substituted value. $- or -$ - prefix or suffix indicating that an alphanumeric string is hexadecimal (presented in base 16.) The J1979 specifications which establish the OBD II protocol are written using hexadecimal notation throughout. Datastream - a set of PID values, DTCs, test results and/or PDTCs; the display of such data on or via a scan tool. Freeze frame - a set of PID values indicating then-current data written into the PCM’s memory when a DTC
loss of my notes from that vehicle, and I can no longer remember even which foreign nameplate make it was, much less the year, model and engine. What I do remember is that it sure threw me for a loop! I also remember rechecking this at the time with another scan tool with the same result, so I suspect that it was simply the result of a mistranslation somewhere along the way, and not a tool glitch per se.) One more note on the commanded equivalence ratios PID: You’ll find it in use for diesels as well. Stoichiometric conditions for gasoline engines result in an air/fuel ratio of approximately 14.7:1. The advent of oxygenated fuels has accustomed us to seeing lambda values showing slightly lean, up to as high as 1.04 in some cases, with no apparent fault. Since fuel blends vary both regionally and seasonally, normal values for your area may differ. With diesels, the ratio is closer to 14.5:1, with propane running best at 15.7:1 and natural gas working out to about 17.2:1. If you’re using your gas analyzer on a vehicle burning one of these fuels, you’ll have to reset your lambda calculations accordingly. Most gas analyzers with a computer plotting interface readily accommodate multiple fuel types, usually from the setup menu. In the case of flex-fuel sets, similar to an aircraft flight recorder. Note: Freeze frame data is erased when codes are cleared; be sure to read and record before clearing DTCs. CAN - controller area network; also, communication via the same. Monitor - one or more self-tests executed by the OBD system to determine whether a specific subsystem is functioning within normal limits. Monitor status changes to incomplete or “not done” when DTCs are cleared, and returns to complete or “done” once all relevant self-tests have been run. A monitor status showing completion is not a guarantee of a successful repair unless there are no codes and no pending codes, and unless the vehicle has been operated under conditions similar to those under which a previous fault had occurred (see freeze frame).
DOING IT ALL WITH GENERIC DATASTREAM cars, check the ETOH_PCT PID to help your analyzer figure out the correct stoichiometric ratio. Once you’ve made the proper selection, you can work from lambda without bothering to know or remember the exact stoichiometric ratio involved. I was certainly glad to see the appear-
ance of purge data in the generic list, as knowing the commanded purge status can assist in diagnosing several types of driveability faults above and beyond evap leaks and malfunctions. Remember, however, that this PID reflects only the current commanded state, not necessarily what’s actually happening.
The PIDs for EGR Command and EGR Error are likewise helpful. Depending on the interface you use, however, EGR_Error may be reported “backwards,” with 100% indicating that command and position are in complete agreement and 0% indicating that one shows wide-open while the other shows shut. (I’ve seen this on numerous Hondas, where a 99.5% “error” actually meant that the valve was closed as commanded.) As usual, a few minutes checking known-good vehicles can help avoid many wasted hours hunting problems that aren’t really there. Other new PIDs inform us of the mileage since the last time the codes were cleared as well as the distance driven since the MIL first illuminated for any current codes. Both of these pieces of information can be useful, especially if yours is not the first shop to look at a particular problem. In the case of intermittent faults, they can also help give you a better idea of just how frequently the issue does arise.
Beyond PIDS
Circle #16
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July 2014
Potentially both more helpful and more problematic are the new Permanent DTCs found in mode $0A. These cannot be cleared directly via a scan tool, but will be self-erased once the corresponding monitors have successfully run to completion. Attempts to circumvent plug & play emissions tests by simply clearing codes without fixing the underlying causes led to the development of these Permanent DTCs. While there are times when I would rather just “kill the MIL,” the PDTCs make me take the extra time to more fully educate my customers and to verify the efficacy of my repairs, often by resorting to Mode $06 data analysis. The key thing to remember when working with Mode $06 data is that it’s entirely up to the OEM to define all TID$, CID$, MID$, etc. These definitions can vary by year, engine, model and/or equipment even within the same OEM division, so be sure to verify the accuracy of any information you’re using to interpret this data before you get yourself in trouble. Also remember that many manufacturers
populate their Mode $06 datastream with “placeholder” values after codes are cleared and until affected monitors have run to completion. This is a strong argument for waiting as long as possible before clearing codes. Probably 90% of the MIL-on complaints we see in my shop are resolved using “just” a generic scanner, coupled, of course, with a few decades of experience! Nevertheless, since a generic scan interface can take you only so far, there are certainly other times when we break out one of our more sophisticated scan tools with bidirectional functionality, access to additional PIDs, guided diagnostics, etc. Especially in an older vehicle, the generic communications data rate (baud speed) may also seem slow by today’s standards. After an initial scan, this limitation can often be overcome by selecting a relatively small number of PIDs relevant to the problem at hand. All vehicles since 2008 support CAN communications even in the generic interface. The effective data transfer rates
here are plenty quick enough for almost any practical purpose. Since OBD II generic standards do not apply beyond P-codes (and some Ucodes), any full-service shop needs one or more scanners to deal with B-, Cand most U-code issues. Remember, though, that many OEMs illuminate TRAC, VSC and/or ABS lights in response to any P-code. This is nearly universally true in the case of drive-by-wire (electronic throttle body) applications, but may be found in many other instances as well. In all such cases, you must resolve the P-code issue first, before worrying about any of these sideeffects codes. If you have an appropriate interface, once you’ve killed the MIL, clear those extra codes as well, so the next tech doesn’t find them still in memory if and when a legitimate B- or C-code ever does set. The bottom line is that there are several potentially important advantages to using a generic scan interface for initial code retrieval and data analysis, so don’t be afraid to get your feet wet! Since the
generic datastream focuses on the most important inputs and commands, where the bulk of problems occur, and since all PID values reflect their associated sensor states without substitution, you’re less likely to be capsized by a flood of irrelevant data. As always, checking known-good vehicles will help keep you on an even keel and familiarize you with what “good” looks like. While you may occasionally wind up switching over to an enhanced interface, you’ll likely find that routinely starting in generic using a fast and inexpensive basic scanner results in much greater efficiency. Whether your shop is large or small, this practice also lets you avoid excessive wear and tear on the more expensive and advanced scanners and keeps them free for those longer-term diagnostic challenges where their enhanced features are actually needed. This article can be found online at www.motormagazine.com.
Circle #17
July 2014
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Driveability Corner New PIDs provide additional information that can be included in your diagnostic efforts. But before it can be used, you must understand how it was obtained and what it’s intended to represent.
Mark Warren
PID—there is no differentiation for Bank1 and Bank2. Now let’s get into the layout of the screen capture below, from the 2010 Tacoma: In the top chart, rpm is in red (the scale on the left-hand side) and vehicle speed is in green (the scale on the right-hand side). In the second chart, air/fuel ratio sensors Bank1 and Bank2 are in milliamps, and both are scaled on the left-hand side. In the third chart, the air/fuel ratio sensors Bank1 and Bank2 are in volts, and both are scaled on the left-hand side. In the fourth chart. postcatalyst O 2 Sensors Bank1 and Bank2 are in volts, and both are scaled on the left-hand side. In the last chart, the commanded EQ ratio is scaled on the left-hand side. All data to the left of the dotted line in the screen capture is a baseline test drive ending with a long idle period prior to introducing a skew in B1S1 AFR sensor (the ver-
Screen capture & chart: Mark Warren
[email protected]
T
he commanded equivalence (EQ) ratio parameter (PID) is required in the generic datastream on all passenger vehicles since 2008 (see the EQ to air/fuel ratio [AFR] matrix on the next page and the SAE definition in the box on page 13). An EQ of 1 equals 14.7:1 AFR. This PID should reflect the commanded air/fuel ratio. That being said, there are no Bank1 and Bank2 EQ ratio PIDs, and can’t the EQ or AFR be different for each bank? Are the two averaged? Yipes! How is this going to work on a vehicle? This test was performed on a 2010 Toyota Tacoma with a 4.0L engine. It’s important to note that this may not be representative of other vehicles; this is one test. Also, this test is not intended to be critical of any implementation of the EQ PID. I think the problem is in the original definition of this
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DRIVEABILITY CORNER FEB 2015v2_Layout 1 1/22/15 10:22 AM Page 2
tical dotted line). The solid green line is the point of measurement for the reading in the small boxes to the right of the parameter name on the chart. Finally, I’ve drawn a solid fine black line horizontally in the EQ chart to show EQ equals 1.
amount of amperage used and the conversion to volts scaling. Note the postcatalyst oxygen sensors also following each other reasonably close-
ly. It’s noteworthy that at 105,000 miles, the rear O2 sensors don’t go above .8V and lay flat on zero for some period of time. Perhaps these continued on page 13
Data Analysis Remember that the air/fuel ratio sensors (charts 2 and 3) read high when lean and low when rich—the opposite of the O2 sensors that are
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Driveability Corner sensors are showing some age. Note that the EQ PID in the baseline data period is pretty active when the truck is being driven. Look at the EQ relative to the top
At 105,000 miles, the rear O2 sensors don’t go above .8V and lay flat on zero for some period of time. chart of rpm and mph to get an idea of the changing load. Notice that the fuel-cut events that are reflected well in the AFR, and O2 sensor
SAE Definition: Commanded EQ Ratio Fuel systems that utilize conventional oxygen sensors shall display the commanded open-loop equivalence ratio while the fuel control system is in open loop. EQ_RAT shall indicate 1.0 while in closed-loop fuel. Fuel systems that utilize widerange/linear oxygen sensors shall display the commanded equivalence ratio in both open-loop and closedloop operation. To obtain the actual A/F ratio being commanded, multiply the stoichiometric A/F ratio by the equivalence ratio. For example, for gasoline, stoichiometric is 14.64:1 ratio. If the fuel control system was commanding a .95 EQ_RAT, the commanded A/F ratio to the engine would be 14.64 x 0.95 = 13.9 A/F ratio.
data are not well represented in the EQ data. The EQ ratio data looks almost backwards when compared to the rich periods on the AFR’s (low) and the O 2 ’s (high). The EQ looks like it’s going in the opposite direction. Okay, now let’s look at the point of defect. I skewed the B1S1 AFR sensor. You can see the immediate skew in the AFR sensor and O2 sensor data. The O2 sensor rails at the bottom (lean). The AFR B1S1 initially skews down, recovers at idle and then skews down again under load. The AFR sensor is skewed to look rich, a false signal I created. The fuel response is to react to lean the “rich” mixture. The rear O2 sensor shows the enleanment and the EQ shows the command to lean. Is the EQ using just Bank1? Is it an average of both banks? Right now I have more questions than answers. I’ll skew Bank2 next time and see where it leads.
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Trouble Shooter One definition of insanity is repeating the same action and expecting a different outcome. After multiple replacements of the same part, it may be saner to look elsewhere for the cause of the failure. Déjà Vu All Over Again
Karl Seyfert
were the same (reduced power) and the same DTC P0121 was stored in memory. I followed all of the recommended diagnostic procedures, then replaced the throttle body (a second time). We recently heard from the customer, and the vehicle is apparently experiencing the all-toofamiliar reduced power symptoms and the Check Engine light is on. Is it time to install a new OE part? I have not previously had any problem with reman parts purchased from this supplier. Is there an underlying issue that is shortening the throttle body’s life span? I don’t want to throw any more of the customer’s money at this problem without finding an answer. Jerry Burns Trenton, NJ Due to the large amount of time that has elapsed between each failure occurrence, we’d have to consider this to be a very intermittent
Photo: Karl Seyfert
[email protected]
A 2007 Chevy Impala with a 3.5L engine came into our shop for the first time about a year and a half ago. The MIL was on, the engine had reduced power and DTC P0121 (Throttle Position Sensor 1 Performance) was stored in the PCM memory. I removed, cleaned and remounted the throttle body, then flashed the PCM and inspected the wiring harness. The DTC did not return, so the vehicle was returned to the customer. About six months later, the throttle body failed with the same DTC P0121 stored. At this time the throttle body was replaced with a remanufactured aftermarket part. The wiring harness was also rerouted, as it did not appear to have enough slack between the body and the engine. Fast-forward another six months or so and the throttle body failed again. The symptoms
Perhaps due to safety considerations, this 2007 Chevy Impala throttle-by-wire throttle housing has “no user serviceable parts inside.” Any accumulated gunk can be removed from the area around the throttle blade, but familiar adjustments to components like the TP sensor are no longer possible.
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Circle #6
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problem. Intermittents are certainly more difficult, but not impossible, to diagnose. Perhaps the most helpful information that could be used to solve this problem would be the freeze frame data. This would tell you the operating conditions at or near the moment when the DTC P0121 was stored. Were the freeze frame data parameters the same (or similar) each time the DTC was stored? If they were, is it because more than one throttle position sensor has failed in exactly the same way? This scenario is not impossible, but it seems statistically unlikely, unless a series of faulty parts were involved. In general terms, what do we know about the possible causes of a P0121? It begins when the PCM detects a malfunction that’s causing an excessively low or high voltage signal to be sent from throttle position sensor to the PCM. This can be caused by a throttle position sensor that has an internal fault. Since the sensor can’t be replaced separately, this is probably why you’ve been installing replacement throttle body assemblies. The DTC can also be caused by a throttle position sensor harness that’s open or shorted. A poor or intermittent electrical connection in the throttle position sensor circuit could also be to blame. Lastly, and probably the least likely, the PCM may be experiencing intermittent failures. Because this is a throttle-by-wire system, the PCM responds to problems with its inputs by reducing engine power. Under normal conditions, the PCM uses the TP sensor input to detect the actual position of the throttle valve, as well as the opening and closing speed of the throttle valve. If the TP sensor reports that the throttle valve is closed, the PCM would use this information to control other functions, such as fuel cut. If the PCM does not have accurate information about how far open or closed the throttle valve may be at a given moment, it can’t accurately control the opening and closing of the throttle from that point on. The reduced engine power allows the driver to (barely) limp the car into a service facility. This may be an inconvenience,
but should be considered safer than the possibility of a runaway throttle. When the customer brings the vehicle to your shop this time, make certain you capture the freeze frame data before making any changes to the PCM or its programming. When did the DTC store? What was happening at the time? With the original throttle housing still in place, make your best attempt to duplicate these conditions. Monitor the relevant PIDs with your scan tool. To open an even larger window into this problem, attach a digital storage oscilloscope to the TP sensor’s data lines. Watch the scope for any indication of signal abnormalities as you work the throttle through its normal range of movements. This may be a temperature-related failure, so it may be necessary to drive the vehicle long enough to get everything under the hood good and warm. There are a few harness connections between the TP sensor and the PCM. Examine each of them closely for any signs of looseness, fretting or other damage. You mentioned that the harness appeared too tight between the throttle housing and the body. Is it possible that this is or was causing a harness connector to partially separate, causing the TP sensor signal to weaken or intermittently drop out? Once again, manipulating the harness while observing the TP sensor signal on the scope may allow you to capture an intermittent failure. Lastly, there’s your question about the quality of the parts involved, and the possible link to repeated failures. In my research, I found that throttle housing failures are not unheard of on these vehicles, so the original equipment parts certainly are not unbreakable. Some techs have experienced problems with remanufactured replacement parts, while others have not. Before pointing the finger of blame at any replacement part, be it original equipment or aftermarket, new or remanufactured, I’d suggest you first make certain you’ve eliminated all of the other possible problem causes. Installing another throttle housing without doing so might buy you some time, but déjà vu could still be a possibility.
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Trouble Shooter Deciding to replace an expensive emissions control component requires confidence in the accuracy of your diagnosis. Is the decision tougher or easier when you’re working on your own car? Ready to Buy a Cat?
Karl Seyfert
Freeze frame data: Pusheng Chen
[email protected]
and listening for a sound change). I was unable to find any leaks. The BARO reading of 98KPa seems to be in agreement with where I live (Metro Detroit area). I am trying to rule out other possible explanations for the P0420 before having the faith to replace the cat. What puzzles me is why bank 1 is in open loop while bank 2 is in closed loop when P0420 is set. I searched for the most common OBD II codes on an automotive reference website. P0420 was at top of the list (with 13.2% of the total). P0430 was number 10 with 3.2% of the total. I can’t think of a reason why there are significantly more P0420s than P0430s recorded. I think this is interesting. I typically try to spend enough time on a diagnosis until I am confident about making a recommendation to replace any parts. But this P0420 with bank 1 in open loop puzzles me. I have included the freeze frame data that was stored at the same time the DTC was set. I would appreciate your help. Pusheng Chen Novi, MI
I am having a problem with a DTC P0420 that’s puzzling me. The vehicle is my 2009 Cadillac STS, which has about 85,000 miles on it. It’s equipped with a 3.6L direct fuel injection V6 engine, a sixspeed automatic transmission and AWD. The freeze frame data indicates bank 1 was in open loop when the P0420 was set. Bank 2 was in closed loop. I cleared the code and it came back in about 300 miles. Once again, bank 1 was in open loop when the DTC set. During subsequent testing, both bank 1 and bank 2 went into closed loop within a few minutes after engine starts. The B1S1 oxygen sensor switches normally, and responds to throttle (wide-open and closed). Even though it appeared to be functioning normally, I replaced the B1S1 O2 sensor with an OE part anyway, thinking it might be an intermittent O2 sensor. After that, I cleared the code, but it came back again in about 700 miles. I checked for intake leaks (with propane) and exhaust leaks (by plugging the tailpipe with a rag
Freeze frame data is perhaps the most useful information available when attempting to determine the cause an OBD II diagnostic trouble code. This data is collected at the moment the DTC is set and is the next best thing to being there when it happens. What can the data shown here tell us about the P0420 that was set on the 2009 Cadillac STS when it was collected?
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Any DTC that sets only every 300 to 700 miles is going to be tough to diagnose. And one that points to possible replacement of an expensive emissions control component like a catalytic converter when it does is going to be even tougher. So first let me applaud your dedication. And second, thank you for the foresight to save and include the freeze frame data with your note. This data may not provide all of the information we need to reach a diagnostic conclusion, but it should help to get us pointed in the right direction. P0420 is a very popular DTC—or unpopular, depending on how you look at it. It indicates that the PCM has determined that the catalytic converter is performing below an established threshold. OBD II’s number one mission is to keep vehicle emissions as low as possible, and it can’t do that without a properly functioning catalytic converter (or converters, in some cases). OBD II keeps a close eye on converter perform-
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Trouble Shooter ance, and when performance drops below a prescribed level, the PCM will set a DTC. Some might argue that performance levels are set too tightly, making it all too easy for a converter to fail an OBD II monitor. Many vehicles are equipped with 4cylinder engines, which typically have just a single catalytic converter, or one large and one small cat, coupled with a set of pre- and postconverter oxygen sensors situated at either end of the main cat. Your STS is equipped with two of everything because it’s a V6 with separate emissions equipment for each bank. P0430 points to a bank 2 catalytic converter that’s operating below an established threshold. Many vehicles don’t have this second converter, which I believe explains why P0430 is so much further down on the “hit parade” of DTCs, when compared to the charttopping P0420. If all vehicles had two of everything, the two catalyst efficiency DTCs would probably be more evenly ranked in terms of occurrence. The PCM doesn’t have a five-gas exhaust analyzer probe stuck up the tailpipe of your STS, so how does it make the determination that the converter is functioning below the performance threshold? The PCM runs a catalyst monitor test only when certain driving conditions have been met. The engine and converter must be at operating temperature, and the engine may be idling or running under light load at low speed. Your freeze frame data indicates the STS’s engine speed was 1228 rpm. The fuel system should also be in closed-loop fuel control (this is key). There must not be any other unfulfilled criteria or previously stored DTCs that would keep the catalyst efficiency monitor from running. Once the PCM has determined that all preconditions have been met, it temporarily forces the air/fuel mixture rich, to deplete any stored oxygen in the converter. Then the PCM temporarily forces the air/fuel mixture lean to determine how long it takes for the converter to react and for the downstream oxygen sensor to change its switching activity. If the converter takes too long to resume functioning (indicated by postconverter oxygen sensor activity), it means the cat-
alyst is not working efficiently enough to maintain the vehicle’s emissions levels within prescribed limits. OBD II will then fail the converter, set a DTC P0420 and turn on the Check Engine light. I believe your vehicle is setting a DTC P0420 only every 300 to 700 miles because that’s how long it takes for all of the preconditions to be met, and for the PCM to run the catalyst efficiency monitor. Alternately, the monitor may be running more frequently, and failing only once every 300 to 700 miles. The key piece of information contained in the freeze frame data is the indication that bank 1 was in open loop at the time the freeze frame data was stored. On the face of it, this makes no sense, as the catalyst efficiency monitor should never have run in the first place with half of the fuel system still in open loop. Achieving closed loop is one of the first preconditions the fuel system would have to satisfy before the PCM would even consider running the catalyst efficiency monitor. We know that freeze frame data is stored at the moment the PCM decides to flag a DTC. So in this case the data was probably collected a certain period of time after the PCM attempted to run the catalyst efficiency monitor. The fuel system had to be in closed loop when the monitor began to run, but something happened after that, and it was no longer in closed loop when the freeze frame data was stored. This is a hypothesis, as we don’t know how quickly the PCM updates the freeze frame data we’re now using for our diagnosis. I’d suggest you look for a component that’s capable of intermittently kicking the fuel system out of closed loop. This may be happening at other times, besides when the PCM is attempting to run the catalyst efficiency monitor. Besides the pre- and postcatalyst oxygen sensors, most of the other input sensors have their own OBD II DTCs that should give you an indication of a problem. But it may be too intermittent to trigger a DTC and the only way you may be able to identify it is by monitoring a limited set of PIDs, waiting for the glitch to reveal itself. It can’t hide forever, and you’ve already shown that you have the patience to wait.
October 2014
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OBD II Code Diagnosis Part III
Bulletin TB-80035 September, 2011
Gary Stamberger – Training Director Magnaflow Exhaust Products In the first part of this OBD II Code Diagnosis series I stated that we would discuss the principles of OBD II codes and breakdown each character that defines them. For a generic discussion of OBD I’ll refer you to TB-80016 and 80017. We archive all of our bulletins and they can be found on our website at www.maganaflow.com. Look for Tech Bulletins under Tech Support. For this series I would like to stay on a more specific path. In our first two parts we took a very common Ford EGR code and broke down the diagnosis. I chose this code not only for its commonality but also because this EGR system uses several components, each one playing a major role in the vehicles ability to reduce NOx. Although the PCM has the ability to set several different and distinct codes for each component (9 generic and 10 specific) the interrelation of the components cannot be ignored. As we saw in our example, one of the possible causes for the P0401 code was mechanical and had nothing to do with the malfunction of any one component. Another common issue in Code Diagnostics sometimes overlooked is that of retrieving codes in both OBD II Generic and Enhanced or Manufacture Specific mode. Depending on the tool being used, the enhanced option may not be available (i.e. Code Reader only). Using generic mode requires less input therefore is faster and in most cases will get the technician to where he wants to be. The downside is that it is a generic code and therefore in many cases the repair information will not be specific to that vehicle. The obvious upside then to using Enhanced Mode, is that the diagnostic information will be specific to that vehicle or at least that manufacturer. The description and operation will give you a better idea of what the PCM is looking for and the subsequent testing should lead you to the proper diagnosis the first time. Example: 2005 Altima, 2.5L with an illuminated MIL. The OBD II code was P0140, O2 Circuit B1S2 No Activity Detected. A quick glance at the data stream showed that under the proper test conditions the sensor displayed activity. At this point we might determine that it is an intermittent problem, clear the code and send the customer on their way. However a look at Enhanced codes revealed a P1147, O2 B1S2 Maximum Voltage not Obtained. A closer look at data stream showed that the sensor was not reaching a specific maximum voltage of .78v. This specific information was not available when processing the P0140 code. The key to any diagnostic situation is to always follow a pattern for each problem we face and code diagnostics is no different. Yes… each manufacture has common problems and knowing where to find that information is valuable but sometimes even the “silver bullet” can be a dud! Whether it is a no start, misfire, won’t idle, MIL illuminated or any number of issues, having a plan is by far the best plan. “Shot Gun” diagnosis will on occasion allow us to hit the illusive homerun but more often than not we spend a whole day repairing a component only to go home with that empty feeling in our stomachs, knowing the same problem will reoccur in the morning. Diagnostics is an art and getting good at it can be a great confidence booster, however these vehicles are changing constantly and there is no time to rest. As I say when closing all my classes: THE RULES ARE ALWAYS CHANGING TECHNOLOGY KEEPS MOVING FORWARD EDUCATION IS A CONTINUAL PROCESS
Cleaning up the environment…one converter at a time Gary
Bulletin TB-80017 December, 2009
On Board Diagnostics Part II Gary Stamberger – Training Director Magnaflow Exhaust Products As promised from last month, more on OBD. Refer to our Website, Magnaflow.com for archived Bulletins. (http://www.magnaflow.com/07techtips/techbulletins.asp) Data Stream Referred to as Current Data or Live Data, this information is available to the technician using a Scan Tool. The number of PIDS (Parameter Identification) available at any given time will depend on a couple of different factors. The particular vehicle (Manufacturer) involved will have the greatest influence on the amount of data available. Followed by the type of Scan Tool used and whether you are viewing the data on the Global OBD II side or Manufacture Specific, aka Enhanced Mode. (Figure 1) Most Scan Tools will have options for viewing the data in different formats such as digital or graphing mode. Graphing can be particularly useful when looking at Oxygen Sensor activity. (Figure 2) The data available will consist of inputs and outputs, calculated values and system status information. Viewing data and becoming proficient at recognizing problem areas is one of the skills we spoke of in last months Bulletin (TB-80016). Part of any training on a particular tool is the repetitive process of using it over and over until you begin to recognize when certain data doesn’t look right. This process will then lead you toward a problem area where further testing will reveal the fault. You can not recognize bad data until you have looked at enough good data. One item to be aware of is the practice of substituting good data values for suspect ones. Due to something called Adaptive Strategy, when the PCM suspects that a particular input may not be reporting accurately, it will substitute a known good value for that sensor and run the vehicle on learned values. This will only show up in Enhanced Mode as Global OBD II will always display actual values. This should not deter you from viewing in Enhanced Mode. It has always been my practice to look at codes and data in both modes.
FIGURE 1
FIGURE 2
Freeze Frame Freeze frame is a “snap shot” of data taken when a code is set. This can be very valuable information as it allows the technician an opportunity to duplicate the conditions under which the trouble code was recorded. The number of freeze frame events recorded and viewable by the technician will again depend on the vehicle and scan tool being used. Early systems could only store one batch of information, if more than one code was recorded we would typically only be able to view the Freeze Frame for the last code set. Changes in both OBD and Scan Tool technology have allowed us to have multiple sets of information available for multiple codes set. One exception is that of Misfire. Misfire codes and subsequent data take precedent and will overwrite any previous freeze data stored. Be aware that all freeze frame information is lost when codes are cleared.
FIGURE 3
FIGURE 4
Courtesy Toyota
Monitors Monitors, also referred to as Readiness Indicators are considered the single most comprehensive change that came with OBD II. CARB and the EPA recognized that a vehicle started polluting long before the PCM recognized a fault, set a code and illuminated the MIL. Early OBD systems did not have the capability to recognize degradation of components or systems. Today’s OBD II system is designed to recognize when a vehicle could potentially exceed its designed emission standard by a factor of 1.5. It does this through a series of system Monitors. During normal operation the PCM will conduct certain tests to gauge the operational health of a particular system or component. The Monitors operate in two categories, Continuous and Non-Continuous. As you can probably guess the Continuous Monitors run, well, continuously. They are Misfire, Fuel System and Comprehensive Component. Non-Continuous consist of Catalyst, Evaporative, Oxygen Sensor, Oxygen Sensor Heater, EGR Monitor and more. These require a very specific Drive Cycle (Figure 4) that will meet all the criteria necessary for a complete test. Scan Tools will have a Monitor Status screen that indicates if the Monitors have run to completion. (Figure 3) Next to each component or system it will indicate “Ready” or “Not Ready”, “Complete” or “Incomplete”. If the vehicle is not equipped with a certain system the screen will indicate “Not Supported” or “Not Available”. When one or more indicators read Not Ready or Incomplete, it is an indication that codes have been cleared recently, either with a scan tool or loss of power to the PCM such as battery disconnect. If there is no history of either of these events occurring this is an indication of the PCM intermittently loosing power or it is rebooting which could be an internal problem. It is commonly known that the Catalyst and Evaporative System Monitors are the hardest to run to completion. Many states have moved to an OBD system test for Emission Testing in place of tail pipe testing for vehicles 1996 and newer. California is considering this transition as we move into 2010 (No date has been set for implementation). The test includes checking for proper location of DLC (Data Link Connector), bulb check of MIL, no MIL when vehicle is running, no codes in system and all the Monitors have run to completion. Monitors are a key component because they are a direct indication of whether the OBD system had been tampered with prior to Inspection. The USEPA and CARB authorities have generally found that OBD II systems are more effective in detecting emission-related malfunctions on in-use vehicles compared to existing Inspection and Maintenance (I/M) tailpipe testing procedures. Current Smog Check data indicates that vehicles are more likely to fail an OBD II-based inspection than the required tailpipe emissions test. With the reduced testing times (10 mins. for OBD vs. 20 mins. for tail pipe) and cost savings in equipment it’s not beyond the realm of possibility that states currently having none or minimal Inspection Programs may consider adopting an OBD Emissions Testing program. These programs have proven to create a healthy environment and also a healthy bottom line for repair shops.
Cleaning up the environment…one converter at a time Gary
Bulletin TB-80010 May, 2009
INTERPRETING FUEL TRIM DATA
Gary Stamberger – Training Director Car-Sound/Magnaflow Performance Exhaust This month we take the discussion of Oxygen Sensors to yet another level. In recent discussions we talked about the role these sensors played in closed loop fuel control. What exactly does that mean, “Closed loop fuel control”, and what role does it play in maintaining a good working converter? When a vehicle is started cold there is a warm up period which is referred to as, “Open loop”. It’s during this time period that the engine is polluting the most. Consequently, getting to closed loop fuel control is a top priority. The PCM has an internal clock that restarts on each start-up and it knows, based mainly on temperature, how long before all components are operating and it is ready to enter closed loop. To this end, many elements have been added to the systems. Oxygen sensors have built in heaters to speed the warm up process. The PCM can detect when the engine is taking too long to come up to temperature and will set a code P0125, “Insufficient temperature for closed loop fuel control” which typically means the thermostat is stuck open. Once the conditions are met and the PCM gains fuel control the goal then becomes maintaining it. The oxygen sensor is referred to as a, “Voltage Generator” and reports the content of oxygen in the exhaust stream to the PCM ranging between 100mv (Millivolts) and 900mv. When the oxygen content is high, (Voltage is low, near 100mv) the PCM sees this as a lean condition and its response is to add fuel. When the sensor reports back that there is little oxygen in the exhaust stream (high voltage, near 900mv), a rich condition is sensed and the PCM pulls fuel away. A technician can monitor this data on a scan tool as, “Short Term Fuel Trim” or STFT. A positive percentage indicates the computer is adding fuel while a negative number says it is taking fuel away. If the PCM is in fuel control, monitoring the direct relationship between O2 and STFT scan data will confirm it.
The next step then is to look at Long Term Fuel Trim (LTFT) percentages. These numbers give us a history of what the PCM has been doing with fuel trim over the long haul. As with STFT, positive percentages tell us the tendency is to be adding fuel (compensating for a lean condition) while negative numbers indicate the PCM is pulling fuel back, (Overcoming a rich condition). If either of these conditions exists for a prolonged period of time and the LTFT percentages exceed the PCM’s parameters a fuel trim code will set (P0170-P0175) and Check Engine light illuminated. The example below shows us that although the PCM appears to be in fuel control there is evidence that it has been adding fuel over time.
Our concern when looking at fuel trim is what it may be telling us about engine efficiency and whether the computer has been compensating for other fuel related problems. If the engine has been over-fueling the question is…WHY? A leaking fuel injector, fuel pressure regulator, lazy O2, or bad Mass Air Flow (MAF) would be some of the considerations. The same issue exists if it’s too lean. Here an air leak, clogged injectors or fuel filter, or miscalculated air flow could be the cause. Any Fuel Trim condition that persists will eventually take its toll on the catalytic converter and must be addressed by the repair technician before installing a new one.
Cleaning up the environment…one converter at a time
Gary
November 2006 Premier Issue
Practical uses of Mode $06 Round out your diagnostic skills By Phil Fournier
Practical uses of Mode $06, by Phil Fournier Page 1 of 1
November 2006 Premier Issue
As I worked to get a handle on the presentation of Mode 6 at a technician’s level, I was reminded of a slide in one of my PowerPoint presentations that announces “the lab scope allows the technician a look inside the manufacturer’s electronic strategy.” I’m going to make a similar statement here regarding the use of Mode 6. Mode 6 gives the technician a look inside the manufacturer’s strategy. Sometimes it’s a blurry look, and sometimes it’s a look of limited usefulness, but taking the look is worth the trouble. It will make the technician who takes the trouble a better-rounded diagnostician, even if he/she only uses it for a few selected items.
What It Is First off we have to cover some basics for those still uncertain of what mode 6 is or isn’t. Mode 6 is part of the SAE standards that defined what kind of data would be available to technicians through the OBD2 interface. Simply put, it is the brains behind the operation of the OBD2 monitors of various emission control systems. In theory, it covers what we know as the non-continuous monitors, those usually run by the OBD2 system one per trip if the conditions are right. By now, we all know that those include Fuel Evap, Catalyst, O2 sensor, O2 sensor heater, EGR, and so forth. But the cool thing about the information available in some mode 6 data is that it breaks down the monitor into its various parts, sometimes giving us useful information that cannot be seen as well through looking at live data stream or looking at stored trouble codes. I’m going to start off by suggesting that if you are serious about learning the benefits of Mode 6, invest a few bucks in a scan tool capable of doing the interpretation for you. If you don’t know what I mean by that, it means you are not currently using Mode 6. The first time I stumbled across Mode 6 data was while randomly pushing buttons on my scan tool and looking at stuff. I rapidly backed out of the screen due to what looked to me to be completely useless information, filled with $ signs, things called TID’s and CID’s, plus letters and numbers mixed. And so it is unless you have a way to interpret the data. This is because Mode 6 was written in Hexadecimal code (Base 16 instead of Base 10) and not particularly designed with the technician in mind. But never mind, there are plenty of things we have learned to use that fall into this same category.
Hex to Key Because this article is designed to be useful information, I don’t want to get bogged down in a boring discussion of Hexadecimal numbers and Base 16. I will just suggest though that you can use your free Windows calculator to convert the letters and numbers into pure numbers. If you use “Scientific” from the “View” menu, you will get a choice of the “Decimal” calculator or the “Hexadecimal” calculator. Entering the letter and number combination into the Hexadecimal screen and then clicking the button for Decimal will convert the number to a readable number. Unfortunately, that number will still do you
Practical uses of Mode $06, by Phil Fournier Page 2 of 2
November 2006 Premier Issue little good unless you know what it means via a conversion key. Part of the reason that this article is based on Ford vehicles is because the Ford information is available for free at www.motorcraftservice.com. Choose “OBD2 Theory and Operation” from the menu on the left of the screen, then scroll down and pick the Adobe Acrobat file to open. Once the file is open you can go to the monitor that you want to look at.
Misfire Counts But leaving all that behind, there is one area of Mode 6 on a Ford which you can start using immediately, as long as you have some scan tool that will display Mode 6 (and not all of them do; I’ll include a list later in the article of assorted scan tools and how to find Mode 6 in them.) I refer to misfire counts, which are not contained in Ford’s regular Enhanced data stream on the majority of scan tools. Test ID $51 in 1996-98 vehicles and Test ID $53 from 1997 and on in others will display misfire counts WITH THE CORRESPONDING CYLINDER shown as the Component ID ($01 through $0A). Note that $0A is Hex code for the number 10 and indicates cylinder number 10. If you have less than ten cylinders, you can ignore the data in any of the CID’s above your number of cylinders because it is bogus. But the beauty of this data is that at last you can identify which cylinder has misfire counts in it, especially when you have NO CODE. The reason for this can be seen in the data. Look at the figure below, captured off a 1999 Merc Marquis with no codes, but a complaint of a misfire under certain load conditions. Note that I am using the AutoEnginuity PC based Scan tool that interprets the data for us but you can most likely use your own scanner in a similar fashion.
Notice that all cylinders have zeros in the misfire counts column except cylinder #3. The counts are low, nowhere near the threshold required to set a code. But this information was invaluable on a coil-on-plug engine that I had no way to connect to in looking for a misfire under load condition. After inspecting the plug boot and spark plug for any sign of arching, and finding none, all I had to do to verify the coil was failing was to swap it with another cylinder and take the car for a second road test, after clearing codes. (Note: Though there were no codes, a code clear procedure removes the data from Mode 6 misfire monitors; this is not necessarily true of all data in Mode 6 though.) Finding the misfire has followed the coil to a different cylinder, the coil can be replaced with perfect confidence in a proper repair.
Practical uses of Mode $06, by Phil Fournier Page 3 of 3
November 2006 Premier Issue It is a bit ironic that Mode 6 data on a Ford contains misfire counts, since SAE J1979 defined Mode 6 as non-continuous monitor information and Mode 7 as continuous. Never mind, we’ll take the data for its usefulness, even if it is in the wrong place. But this little snafu is symptomatic of Mode 6 data in general. Not all things are as they might be expected to be, which makes some technicians throw up their hands and conclude that the moving target is not worth the trouble. But let’s carry on and see what other use we might get out of it.
Cat Stats I know many technicians who sweat over the replacement of a catalyst because of a code P0420/P0430. When factory cats cost in excess of $800 each, it is small wonder that they worry about a misdiagnosis. But how about if we could record the Mode 6 data, clear the code, and then drive the car to see what resets in the Mode 6 monitor? Let’s see what our 1999 Grand Marquis with 108k miles on it shows for Mode 6 data on its catalyst monitor.
This cat monitor was run after the coil was replaced, and we can safely conclude by looking at the data that this catalyst is still in good condition. The switch ratio of the bank 1 cat (the side of the misfiring coil) is actually a bit better than that of bank 2. And both cats are well under the limit described here as .842, which according to the Ford website reflects a limit of the percentage of switching of the rear O2 sensor as opposed to the front one. Notice what happens to the data when we clear codes:
It didn’t go to zero, did it? Instead, some random number got put in the box, a number that looks like a near-failure of .749. But what’s the likelihood that both cats would measure the exact same switch ratio? Just about zero, but this illustrates the need to not be careless in your treatment of Mode 6 information. The graphic below (captured from a 1998 Ford Winstar) show what the data would like uninterpreted:
See Chart on following page!
Practical uses of Mode $06, by Phil Fournier Page 4 of 4
November 2006 Premier Issue
You could do the math, converting the numbers from Hex to Decimal, and then multiplying by the conversion factor listed on the website of .0156. But why bother? It is easy to see that the two catalysts on this vehicle are well within the maximum limit. But what if you had a code P0420, cleared it, then drove the vehicle and saw the 10/11 test number at 19? You could be pretty comfortable at recommending the catalyst, and depending on where you saw the 10/21 parameter, you might be recommending a pair.
On the following pages… Restricted EGR and scan tool tips
Practical uses of Mode $06, by Phil Fournier Page 5 of 5
November 2006 Premier Issue Choked off? Restricted EGR is one other place where Mode 6 on a Ford can come in handy. Here is data from a normal system (1999 Ford Crown Vic 4.6L with no problems):
Notice that the TID $45/$20 is the same as the DPFE voltage as long as the EGR valve is not stuck open. This can help a technician that may not have enhanced Ford data, as the generic data stream (Mode 1) does not list DPFE voltage as a parameter. Listed below is the data (same vehicle) where I disconnected the intake side hose on the inconveniently located DPFE (see photo ?):
Note that this set a pending code P0401(EGR flow insufficient) on a single road test. However, because we have no specs for TID 41/CID11 & 12, they do not help us like they are supposed to and the vehicle does not set the code P1405 like it is supposed to. But obviously something is detected. Next I reconnected the DPFE hose and installed a restriction in the EGR valve (see figure ?) to simulate a restricted EGR passage. I captured the following data:
I find it very interesting that TID $4A/$30 has barely passing numbers, but in order to achieve that much flow, the computer had to ratchet up the EVR duty cycle to 89%. However, in spite of repeated road tests and completed EGR monitors, this condition would not set even a pending code. Below is another capture with the EGR blocked completely:
Practical uses of Mode $06, by Phil Fournier Page 6 of 6
November 2006 Premier Issue Taken all together, we can see that Mode 6 stored data can help us nail down a restricted EGR passage that does not set a code, now that we know to look at TIDs $4A/$30 and $4B/30. You can probably do that on your scan tool without an interpreter, but you are going to have to do the Hex plus conversion math and use the Ford website. Conclusion In conclusion, Mode 6 is a barrel of data, some of it bogus and meaningless, and much of it powerful. I’m told that Honda recommends the use of Mode 6 data in the diagnosis of fuel evaporative problems. I’d recommend you dive into your scan tool and pick some TID’s and CID’s to figure out so you can get started on learning what to expect, particularly if you have the good fortune to work on a single car line. If you are a multicar line guy like me, try it out on Ford misfires to start with and see if you don’t find it to be a real time saver.
How to access Mode $06 in assorted scan tools (found under Generic or Global OBD II in every case): MasterTech: Select “F5 System Test” then “F2 Other Results” (Note that results are displayed as “Pass/Fail”. To get the actual readings press “*, Help”) Snap On: (later than 2001 cartridge, earlier versions don’t have Mode 6): After communicating with vehicle select “Display Test Parameters/Results” then select “NonContinuous Monitored systems (Mode $6). (MT2500; Solus, Modis similar) BDM: Select “Non-Continuous Monitor Test Results” NGS: Select Diag Monitoring Test Results AutoEnginuity: Select On Board Test Results tab OTC Genesis: Select “Special Tests” then “Component Parameters”
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