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
If 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 II-equipped 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 parameters to cheek 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 operating 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 bank-to-bank hiel control engines. For example, if LTFT B1 is -20% and LTFT B2 is 3%, the source of the problem is associated with Bl cylinders only, and your diagnosis should focus on factors related to Bl 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.
Engine Coolant Temperature (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 should be delivered, to achieve the desired air/fuel mixture. The MAF sensor should he checked for accuracy in various rpm ranges, including wide-open throttle (WOT), and compared with the manufacturer’s recommendations. Mark Warren’s Dec. 2003 Driveahility Corner column covered volumetric efficiency, which should help you with MAF diagnostics. A copy of that article is available at, and an updated volumetric efficiency chart is available at
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 parameter 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 BlSl, 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 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
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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 he using a different fuel control strategy due to an oxygen sensor fault.
ENG RUN TIME = Tune 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 position will display 100%. Keep in mind this parameter does not reflect the quantity of ECR flow-only what the PCM is commanding.
EGR ERROR = EGR_ERR: This parameter is displayed in percentage and represents EGR position errors. The ECR Error is also normalized for all types of EGR systems. The reading is based on a simple formula: (Actual ECiR 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 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.
CAT TMP B1S1/B2S1 = CATEMPIl, 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.
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 batten’. 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_ARS: This parameter is the nonnalized 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 die engine for diagnostic purposes.
OL EQ RATIO = EQ_RAT: Commanded equivalence ratio is used to determine the commanded air/fuel ratio of die engine. For conventional oxygen sensor vehicles, die scan tool should display 1.0 in closed-loop and the PCM-commanded EQ ratio during open-loop. 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 (gt;), 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.