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What You Should Know About Powertrain Control Modules (PCMs)

by Larry Carley copyright AA1Car.com

The onboard computer, or Powertrain Control Module (PCM),is the brains of the engine control system, so when the brain is not functioning correctly neither is the engine or anything else that the microprocessor controls - which may include the charging system, transmission, various emission controls and communications with other onboard control modules. Once a diagnosis has been made (and we emphasize the word diagnosis), then and only then should the PCM be replaced.

PCM Trouble Codes

Onboard Diagnostic (OBD II) diagnostic trouble codes that typically indicate a fault with the powertrain control module include:

P0600....Serial Communication Link

P0601....Internal Control Module Memory Check Sum Error

P0602....Control Module Programming Error

P0603....Internal Control Module Keep Alive Memory (KAM) Error

P0604....Internal Control Module Random Access Memory (RAM) Error

P0605....Internal Control Module Read Only Memory (ROM) Error

P0606....ECM/PCM Processor

P0607....Control Module Performance

P0608....Control Module VSS Output 'A'

P0609....Control Module VSS Output 'B'

P0610....Control Module Vehicle Options Error

If you see any of these codes when diagnosing the vehicle with a code reader or scan tool, it may mean the PCM has failed and must be replaced. Additional diagnostic tests will usually be necessary to confirm the problem is really the powertrain control module and NOT something else. Refer to the OEM diagnostic charts for what these tests are. Usually it involves checking certain inputs to the PCM to see if it outputs the correct response. No response or an incorrect response usually means the PCM is defective and needs to be replaced.

All too often, technicians tend to blame that which they understand least. If an engine is not running right and the cause is not obvious, they may blame the computer. Throwing parts at a problem in an attempt to solve it may be good for the parts business, but when a customer brings a PCM back because it failed to fix their problem, nobody wins. Warranty returns on complicated and expensive components like powertrain control modules are an ongoing problem that costs everyone money.


Over 50 percent of PCMs that are returned under warranty (either because the PCM failed to fix a performance problem or because the engine did not run properly after it was installed) have nothing wrong with them! So it is obvious a lot of people are swapping computers to see if a different PCM will fix their problem.

The trouble with returns is if the PCM has been on the car, you have no way of knowing if it is still "good" or not. Somebody may have crossed up some wires, zapped the PCM with too much voltage or who knows what? The computer needs to be tested and verified before it can go back on the shelf and be sold to somebody else.

Unfortunately, there is no easy way of doing that in a parts store. The PCM has to be hooked up to a sophisticated simulator that exercises all of the computer's input and output circuits to make sure it works correctly - which means the PCM has to go back to the supplier, be retested, and if no fault is found, repackaged and put back into stock.

Be warned, though, that many parts stores have a policy of "no returns or refunds on electronic components."


One way to reduce the risk of PCM warranty problems is to find out why the old PCM died. Determining the cause of death may not always be possible, but it may be essential to prevent the same thing from damaging the replacement PCM in some cases.

PCMs typically fail for one of two reasons: voltage overloads (often due to a short in a solenoid or actuator circuit) or environmental factors (corrosion, thermal stress or vibration). If the shorted solenoid or actuator is not found and repaired, the voltage overload it creates may damage the replacement PCM, too.

As for environmental factors, water is the main thing to avoid. If water gets inside a PCM, it can short circuits and set up irreversible corrosion that ruins electronic connections. Most remanufacturers will not even attempt to repair a PCM if the vehicle it came out of was submerged in a flood. Replacement is the only option. Thermal stress and vibration can form microcracks in circuit boards (which are repairable). This often has more to due with the ruggedness of the circuit design than operation factors in the vehicle itself.

pcm identification label
Identification label on back of General Motors PCM.


Because there are so many different PCMs, accurate identification of the PCM and its correct replacement is absolutely essential to prevent unnecessary returns. Many PCMs appear to be exactly the same on the outside (same sized box and connectors) but may be wired or calibrated differently inside.

If the wrong PCM is installed in a vehicle, it may run but probably will not run well. Close enough is not good enough when it comes to replacing PCMs. It must be the correct replacement for the application.

Accurately identifying the PCM requires not only the vehicle year, make, model and engine size, but also the OEM part number on the PCM itself. Most supplier catalogs list replacement PCMs both ways. So if in doubt, always refer to the OEM number on the PCM and look it up in the suppliers cross reference index to find their replacement part number.

The calibration chip and PROM contains the programming instructions for the vehicle application. That is why it usually does not come with the replacement PCM. There are too many different possibilities. On many newer vehicles, flash memory or "EEPROMs" (Electronically Erasable Program Read Only Memory) are used. If the replacement PROM is not properly programmed for the application, it must be reprogrammed after it has been installed.

Unfortunately, the ability to do this type of reprogramming is not readily available to the aftermarket. The car makers do not want aftermarket technicians messing around with the calibration of their onboard computers because they are afraid doing so may alter emissions or performance. But that is another issue. One such example is Chrysler transmission modules. They must be reset with the factory DRB scan tool and dealer codes to set the "pinion factor," which controls the operation of the speedometer.


Because a powertrain control module can be very expensive to replace, almost all aftermarket replacement PCMs are "remanufactured" units. A PCM is not rebuilt in the same way that an alternator or water pump is rebuilt because there are no mechanical parts that wear out. Remanufacturing in this case usually means testing the powertrain control module, isolating and repairing any faults that may be found, then retesting the powert5ain control module to make sure everything works correctly.

A remanufactured PCM is typically sold one of two ways: on an exchange basis from stock, or on a custom rebuild basis. If a particular PCM is not in stock or is unavailable, you may be able to send an old PCM to a remanufacturer for repair. Turn around time is typically a few days and the cost is about the same as an exchange unit except there is no core charge). The hard part is finding a remanufacturer who can test and repair your powertrain control module.

Some PCMs, though, may not be repairable. As we said earlier, most remanufacturers will not even touch a PCM if it came out of a flooded vehicle.


Replacing a PCM is essentially a matter of swapping boxes assuming the replacement PCM has been preprogrammed correctly for your vehicle application. Accessibility can be a problem on some vehicles because the PCM is often buried under or behind other components in the instrument panel, climate control system or console. Some are located under a seat and require removing the seat.

Regardless of the PCM's location, though, one thing you should do prior to removing the old PCM and installing the replacement PCM is disconnect the battery.

Once the PCM has been installed and reconnected, the battery can be reconnected, too. But the job is not done yet. Many PCMs have to undergo a "relearning" procedure after they have been installed or if they have been disconnected from the battery. On some newer vehicles, a scan tool may be required to reprogram the PCM and to reset the anti-theft system.

On some applications, there may be a specific relearn procedure for establishing the base idle speed and other operating parameters. On others, it may be necessary to take the vehicle for a short test drive so the computer can adjust itself. The exact requirements will be spelled out in the vehicle service manual. The best advice here is to test drive the vehicle after the powertrain control module has been installed. A short drive cycle that includes going over 35 mph will usually reset most PCMs so they will operate properly.

The powertrain control module will also continue to learn and make small adjustments to the fuel mixture and other functions over time as the vehicle accumulates miles. If the PCM also controls the transmission, it may take awhile to relearn the driver's habits so the transmission may not shift exactly the same as before until this occurs.

Finally, if the Malfunction Indicator Lamp (MIL) or Check Engine light comes back on after the PCM has been replaced, it means there is still a problem with the vehicle. The fault is probably NOT the PCM, unless the fault code is for an internal PCM fault. The presence of fault codes means additional diagnosis is required to identify and repair the fault. And until the real problem is found and fixed, the PCM may not function normally.

If the engine control system is not going into closed loop, chances are the coolant sensor or oxygen sensor may not be working properly. If spark timing seems to be over advanced or retarded, the problem may be a faulty MAP sensor, misadjusted throttle position sensor or overly sensitive knock sensor. And if nothing seems to work right, low charging voltage due to a weak alternator or poor battery connections may be the fault.

Remember, a powertrain control module needs all its sensor inputs, proper battery voltage, a good ground and the ability to send out control signals to function normally.

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