Think Like the PCM

Jan. 1, 2020
In every shop I've ever worked in, I've seen techs stumped by what should have been relatively simple repairs. It's not because they didn't have the factory scan tools or because they were less than intelligent. But when the malfunction indicator lam

Use your best diagnostic tool to repair OBD II powertrain diagnostic trouble codes — your head.

In every shop I've ever worked in, I've seen techs stumped by what should have been relatively simple repairs. It's not because they didn't have the factory scan tools or because they were less than intelligent. But when the malfunction indicator lamp (MIL) is on, it's just easier to "send it to the dealer" if a simple part swap or "silver bullet" doesn't cure the problem. It seems that too many of us don't understand how a particular system works and think it's too hard to figure out. So let's see what we can do to change that mindset.

Let me challenge you first with this statement: The powertrain control module (PCM) will only command the MIL to to illuminate if a test it has run has failed. Sometimes it will even rerun the test several times before it alerts the driver that there is a problem. Now who is going to test these systems more thoroughly, you or the PCM?

By understanding the PCM's test strategy and how the systems it tests are supposed to work, you will make your diagnostic routine more efficient and reduce, if not eliminate, your comebacks.


The first step in thinking like the PCM is to understand some basics. Every powertrain diagnostic trouble code (DTC) is associated with a "monitor." A monitor is a test or tests of an emissions-related system or component. Some monitors, specifically the misfire, fuel and comprehensive component monitors run all the time; these are referred to as "continuous" monitors.

Others, such as the evaporative emission system (EVAP), exhaust gas recirculation (EGR) and catalyst efficiency monitors, are run once per drive-cycle and are referred to as "non-continuous" monitors. A drive-cycle is completed when all the conditions needed for a given monitor have been met and the monitor has run to completion. Some monitors need very little time to be completed, others require very specific operating conditions. Each monitor has its own criteria, and its own defined drive cycle.

When a monitor is running, the PCM will look at the results and determine if a fault has occurred. For certain faults, it's programmed to turn on the MIL and record the fault after the first failure. These are the "one-trip" DTCs.

Most DTCs, however, require the PCM to see the same fault occur over two consecutive trips. "Trips" means a completed drive-cycle for the monitor in question. These are the "two-trip" codes and will be recorded as "pending" on the first noted failure. If the same fault occurs the next time the monitor runs to completion, it will "mature" and the PCM will command the MIL to turn on.

If not, the pending code will be cleared. Early OBD II systems will report pending codes in Mode $07 only for failures in the continuous monitors. Later OBD II and controller area network (CAN) systems will report all pending codes in this mode.

The individual tests that make up each monitor can be broken down into three basic types. The PCM can test a component's electrical circuit for opens and for shorts to power or ground. It can also test input devices by comparing values to another component to see if the information from the first makes any sense.

These are "rationality" tests. An example would be the comparison of throttle position sensor (TPS) values to mass airflow (MAF) sensor values to check the TPS.

Finally, it tests the function of output devices and whole systems by operating certain components and looking for the desired result. For instance, the PCM on a Chrysler minivan will command the EGR off when normally on and look for the difference in short-term fuel trim to see if it indeed is flowing properly when open.

Here is where some knowledge and understanding of Mode $06 comes into play. The tests and their results are reported in this mode, and can help in diagnostics by telling you what test(s) failed and by how much.


After verifying the complaint, it's time to hook up the scan tool to the PCM. Because many of us have to rely on aftermarket scan tools, we're going to do the same to show that there are very few powertrain DTCs you can't fix with these tools. To get the most out of your aftermarket scanner, though, make sure it's updated to access Global OBD II modes. The rest of this article is going to be based on using this function to access the information we want to look at.

Before you even connect, look to see if the MIL is on. How many times have you had the complaint of a MIL on, only to see the light is off when you get it in your bay? Remember, think like the PCM.

If the monitor that is responsible for that DTC runs three times in a row without seeing the same failure, the PCM thinks the problem is gone and will turn off the MIL. The offending DTC, however, will remain in memory for a while longer, until a specified number of warm-up cycles has been completed. A warm-up cycle is generally the time it takes to start the car, run it long enough for engine temperature to increase to a certain level and then be shut off. In addition, the PCM will confirm misfire and fuel trim-related codes only under the same conditions as when the fault first triggered a DTC. If the misfire originally occurred under wide open throttle (WOT) at 65 mph, you can't verify your fix at idle in the bay.

The first step after connecting to the vehicle and establishing communication is to check the status of the monitors. Some scan tools will report this information after connection, on others you will need to access Mode $01. What we want to know is if all the monitors have run to completion. Here is where some more understanding of how the PCM thinks is in order.

Monitor status terms vary a bit from tool to tool, but what we are looking for is a "complete" or "ready" indication on all of them. Monitors listed as "NA" or "not available" are not in use by the vehicle you are testing and can be ignored. Monitors listed as "not ready" or "incomplete" can indicate a few things.

First, the drive-cycle criteria may not have been met yet and the monitor isn't finished. Second, a related monitor has issued a "failed" report to the PCM. An example of this is the oxygen (O2) sensor monitor: If the PCM finds a problem with an O2 sensor, then the related catalyst efficiency monitor cannot run accurately and it will not complete.

Once a monitor status has changed to "complete," it will remain that way until it is reset, either by clearing the codes or by disconnecting the battery. This is also important to know. All monitors may show as "complete" but we don't know how old the test results are, especially in the case of non-continuous monitors. You also need to understand that a monitor that shows as "complete" doesn't mean that it didn't detect a failure and the system is OK.

A completed monitor only means that all the conditions necessary for it to run its tests have been met and the tests have been run at least once since being reset.


Let's look at a 2000 Jeep Grand Cherokee with a MIL on complaint. Checking the status of the monitors shows some of the monitors are "not ready." See Figure 2.

From this we can already begin our data gathering for diagnosis. We know that someone has cleared codes and information that may have been stored because there are monitors in a "not ready" status. A valuable resource to use is your service information system for just that – information.

By looking under "Theory and Operation," you can find information on what conditions are needed for these missing monitors to complete. At this point, it may be because a fault is identified in another monitor and that this fault is preventing the others from running. Another possibility is that the driving conditions needed by the uncompleted monitors have not yet been met.

The next clue in this example is that the MIL is illuminated. That tells us that a fault has been de-tected at least once (if it is a one-trip code) because the codes and data were cleared and there have not been the three successive monitors run without the failure the PCM needs to turn off the MIL.

Yet another is in the monitor list. Notice that the oxygen sensor and catalyst efficiency monitors have not completed. Neither has the EVAP monitor. What does that tell you? What do these monitors have in common? The oxygen sensor monitor has to complete before the catalyst monitor can complete. But what needs to happen before the oxygen sensor monitor can run?

The oxygen sensor heater monitor does, and although that monitor also shows as incomplete, it doesn't mean the PCM didn't find a fault. As a part of some monitor strategies, a failed individual test may suspend completion of the monitor. Let's go to Mode $03 and find out. Once you have the codes, resist the urge to clear them. There are still a few more questions for the PCM.

Are there any other faults the PCM found that we haven't been told about yet? Mode $07 may tell us, so let's check for pending codes and see what's there. See Figure 3. How about the conditions that were present at the time the code was recorded? Mode $02 has this information; it is more commonly known as "Freeze Frame Data." See Figure 4.

Freeze frame data is recorded when the PCM decides a fault has occurred and has ordered the MIL to come on. This record will typically show the driving conditions required by non-continuous monitors for their testing and may point out where in the testing sequence the system failed. It is even more useful on continuous monitor DTCs, like misfire and fuel trim, in giving you an idea of the driving conditions that were in place when the fault occurred.

Just keep in mind that the data may have set well after the fault, so use this information to test the vehicle in a manner that will simulate the logged conditions and typical conditions that might have occurred just before the log. For example, a misfire freeze frame might show the vehicle at cruise speed, but the misfire could have happened during acceleration to that speed rather than at a steady cruise.


Now we know what the PCM is thinking. If there is a fault in the heater circuit, the sensor can't be relied on for accuracy. So the PCM will suspend testing of the other two systems until this failure is corrected.

The next step in our procedure is to learn how the PCM operates tests for these codes so we can devise an appropriate test or tests to find the failure. How? By going back to our service information system and looking up the information on the DTCs themselves. We can also research for further information by looking under "System and Component Testing" and the "Theory and Operation" sections.

When you look up the DTC information, you also will find a testing procedure, or "flowchart." I want to challenge you now to rethink how you use the flowchart for your testing. If you don't know or understand why you are performing the tests listed, you may end up with more questions than answers. Instead, think like the PCM and ask yourself "What causes this code to set? What test did the PCM use to determine the fault? How can I test it the same way?"

For these questions, the testing described in the flowchart is a tool to understanding. This is not a complicated example, I know. But the thought process is one you can apply to any system if you do your homework first, and I guarantee a little time invested up front is going to save you more time overall.

A little reading on the DTCs criteria and study of the wiring diagram, tells us that the PCM controls the heater on the ground side. See Figure 5. There is a driver for the heater on each sensor. Power is supplied through a common lead to the heaters themselves. The codes also provide information. These are not rationality or functional tests, but circuit tests.

By studying the conditions for setting the DTCs, we see what the computer is looking for. A reference voltage is sent through the O2 sensor signal circuit, and if the heater is functioning, the resistance internal to the sensor will increase as it's heated. This increased resistance will cause a voltage drop the PCM can see.

If that drop does not reach a specified point in a specified time, the PCM knows the heater is not doing its job. It also continues to monitor the reference for an additional time to make sure the sensor circuit itself is intact. If the voltage remains high, then it's also possible that the sensor is shorted to power or an additional problem exists.

Now it is time to apply our skills as techs. What would cause both of these codes to set? That is how we decide what tests to make. If the heater isn't working, then the sensor will not heat up enough to create the voltage drop the PCM is looking for in both tests. Because the heater monitor must run first, it would make sense to check the heater circuit first.

So far, we've gathered information and tried to consider the process the PCM is using to test its systems. Now that we have it all assembled we can go under the hood and see what's happening, designing the tests we use based on the observations and information we've taken the time to gather. In this particular case, a bad connector pin in the harness for the sensor was the cause. The connector is repaired and we're done, right?


The last step is to verify the repair. Again, who is going to test your repair better than the PCM? This can be done in a few ways. You can research the drive-cycle requirements for the monitor associated with the DTC you repaired and drive the car accordingly.

To do this, clear the codes using Mode $04 on your scan tool. This will reset all monitors as well. Drive the car and recheck the monitor status. If all monitors have completed, you can check for pending codes to see if the fault has returned or if there is one that went untested because its monitor was disabled by the first fault. Remember the "not completed" oxygen sensor and catalyst monitors in this example? It is also a good idea to check the Mode $06 test results for the monitors to make sure that all tests passed. You don't have to be a Mode $06 expert to do this. Just look for test results that look close to their limits or an individual test that is listed as "failed." If you find this, you will have to do some more research to identify exactly what the test is for and what monitor it is a part of.

You can also emulate the PCM tests yourself by learning what tests are involved and how the PCM conducts them. This is also going to take some effort on your part, but as you continue to develop your skills at thinking like the PCM, you'll find the effort gets easier and easier, your comebacks become fewer and fewer, and your customers get happier and happier.

So use the best diagnostic tool you own, your own intelligence, and start thinking like the PCM.
About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels. 

Connect with Pete on LinkedIn.

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