Encourage techs to think like the PCM

Dec. 31, 2019
In every shop I've ever worked, I've seen techs stumped by what should have been relatively simple repairs.

Editor's note: This article was originally published March 2007, so some of the information may be out-of-date. Please use it at your discretion.

In every shop I've ever worked, 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.

Let me challenge your professional customers first with this statement: The powertrain control module (PCM) will only command the MIL 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. Who is going to test these systems more thoroughly, the tech or the PCM? Understanding the PCM's test strategy and how the systems it tests are supposed to work will make a diagnostic routine more efficient and reduce, if not eliminate, your technician customers' comebacks.

Before grabbing a scan tool

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.

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, meaning 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 OBDII systems will report pending codes in Mode $07 only for failures in the continuous monitors. Later OBDII 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. 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 techs what test(s) failed and by how much.

The first step

After verifying the complaint, it's time to hook up the scan tool to the PCM. Because many techs rely on aftermarket scan tools, they're going to do the same to show that there are very few powertrain DTCs that can't be fixed with these tools. To get the most out of an aftermarket scanner, though, it should be updated to access Global OBDII modes. The rest of this article is going to be based on using this function to access the information techs want to look at.

Before even connecting, techs should look to see if the MIL is on (always remembering to 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 until a specified number of warm-up cycles has been completed.

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, the fix can't be verified idling 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, techs will need to access Mode $01. What they want to know is if all the monitors have run to completion. Here is where a greater more understanding of how the PCM thinks is in order.

Monitor status terms vary a bit from tool to tool, but what techs should look 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 being tested 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 the tech doesn't know how old the test results are, especially in the case of non-continuous monitors.

A monitor that shows as "complete" does not mean that it didn't detect a failure and the system is OK. A completed monitor means all the conditions necessary for it to run its tests have been met.

Putting it into practice

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

From this, technicians can begin gathering data for diagnosis. It's evident 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 a service information system.

By looking under "Theory and Operation," techs 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 incomplete monitors have not yet been met.

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

Yet another is in the monitor list. Techs should notice that the oxygen sensor and catalyst efficiency monitors have not completed. Neither has the EVAP monitor. What does that communicate? 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 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 the codes are obtained, I do not recommend clearing them. They're still a few more questions for the PCM.

Are there any other faults the PCM found that the tech hasn't been told about yet? Mode $07 may tell them, so let's check for pending codes and see what's there. 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"

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 a tech an idea of the driving conditions that were in place when the fault occurred.

The data may have set well after the fault, so this information should be used 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.

What's next?

Now it's known 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 the procedure is to learn how the PCM operates tests for these codes so an appropriate test or tests can be devised to find the failure. How? By going back to the service information system and looking up the information on the DTCs themselves.

When looking up the DTC information, a tech will find a testing procedure, or "flowchart," which could lead to more questions than answers. Instead, they should think like the PCM and ask themselves, "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 can help techs understand.

A little reading on the DTC's criteria and study of the wiring diagram reveals that the PCM controls the heater on the ground side. 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, one sees 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.

What would cause both of these codes to set? That is how techs 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, the tech has gathered information and tried to consider the process the PCM is using to test its systems. Now that it's all assembled, it's time for them to go under the hood and see what's happening, designing the tests used based on the observations and information that's been gathered. In this particular case, a bad connector pin in the harness for the sensor was the cause. The connector is repaired and the job is done, right?

Not quite

The last step is to verify the repair. Again, who is going to test the repair better than the PCM? This can be done by researching the drive-cycle requirements for the monitor associated with the DTC repaired and drive the car accordingly.

To do this, advise your professional customers to clear the codes using Mode $04 on the scan tool. This will reset all monitors as well. Drive the car and recheck the monitor status. If all monitors have completed, the tech 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 all tests passed. A tech doesn't have to be a Mode $06 expert to do this. Just have them look for test results that look close to their limits or an individual test that is listed as "failed." If this is found, more research will have to be conducted to identify exactly what the test is for and what monitor it is a part of.

The PCM tests can also be emulated by learning what tests are involved and how the PCM conducts them. This is going to take some effort from the tech, but by thinking like the PCM, efforts get easier, comebacks become fewer and customers are happier.

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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