The Engine Control Module (also called the Powertrain Control Module or PCM) is the brains of the engine management system. It controls the fuel mixture, ignition timing, variable cam timing and emissions control. It constantly monitors emissions performance via its OBD (Onboard Diagnostics) programming, and it oversees the operation of the fuel pump, engine cooling fan and charging system. It also interacts with the transmission controller (if separate), ABS/traction/stability control system, body control module, climate control module and anti-theft system. In short, the engine control module performs a wide variety of functions that are necessary to operate a vehicle.
The electronic components inside an engine control modules are fairly robust, but sometimes things can and do go wrong. Shorts in sensor circuits may overload and damage the module. Problems with the module’s power supply (too much voltage or not enough) or ground connections can cause it to misbehave. Bad inputs from sensors or other modules may also cause it to malfunction. Corrosion, excessive heat and vibration also can cause harm to the module.
When an engine control module fails, it may or may not set any Diagnostic Trouble Codes (DTCs). This can make troubleshooting difficult and often results in misdiagnosis of a bad computer. Often the real problem is outside the computer such as a bad sensor, wiring fault, power relay or voltage issue. Complete module failures are rare, but failures within the module’s various subsystems and memory are more common. A shorted fuel injector, for example, may overload and burn out the injector driver circuit within the engine control module. If the underlying cause is not found and fixed, it can cause the replacement computer to fail, too.
If a bad computer has been accurately diagnosed, replacing it usually requires several steps. First, the replacement computer must be programmed for the specific vehicle into which it will be installed (year, make, model, engine, VIN, trans, etc.). In some cases, the supplier of a remanufactured module can perform this step by providing them with the necessary information. Or, the module can be programmed by the technician using a J2534 pass-through device and scan tool. Programming requires professional expertise and special equipment, and is NOT something a DIYer can do himself.
On older GM and Ford computers (1995 and pre-OBD II), the programming is on a calibration PROM (Program Read Only Memory) chip. The chip has to be removed from the old computer and installed in the replacement computer.
On many vehicles, it is also necessary to perform a relearn procedure after the computer has been installed so the anti-theft system will recognize the new computer otherwise the engine may not start.