The Powertrain Control Module (PCM) controls most all engine and transmission related operations. Its ability to receive information from various sensors, perform rapid calculations and output commands to the ignition and fuel injection allow the engine to achieve low exhaust emissions while maintaining surprising engine performance. The PCM is a complex, sophisticated on-board solid state computer which receives signals from many sources and sensors; it uses these data to make judgements about operating conditions and then control output signals to the fuel and emission systems to match the current requirements. It is factory programmed with the vehicle's Vehicle Identification Number (VIN) to match its computing ability to a specific platform/powertrain/option combination.
Inputs are received from many sources to form a complete picture of Powertrain operating conditions. Some inputs are simply Yes or No messages, such as that from the Park/Neutral switch; the vehicle is either in gear or in Park/Neutral; there are no other choices. Other data is sent in quantitative input, such as engine RPM or coolant temperature. The PCM is programmed to recognize acceptable ranges or combinations of signals and control the outputs to control emissions while providing good driveability and economy. The PCM also monitors some output circuits, making sure that the components function as commanded. For proper engine operation, it is essential that all input and output components function properly and communicate properly with the PCM.
Since the PCM is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a Diagnostic Trouble Code (DTC) is assigned and held in memory. The dashboard Malfunction Indicator Lamp (MIL)-CHECK ENGINE or SERVICE ENGINE SOON-will illuminate to advise the operator that the system has detected a fault.
More than one DTC may be stored. Although not every engine uses every code, there is a large range of codes. Additionally, the same code may carry different meanings relative to each engine or engine family.
In the event of a PCM failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although likely at a reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the PCM enters this mode.