Also referred to as an onboard computer, a powertrain control module (PCM) is a powerful computer that helps in managing and controlling emission systems, fuel injection and the ignition system. In some vehicles, the PCM might also be used to manage the anti-lock brake system and an automatic transmission.
A powertrain control module has two operation modes: open loop and closed loop. Open loop becomes active when the engine is cool and runs on a preset program. Closed loop is active when the engine is running. It monitors the sensors in the system, then it makes any necessary adjustments.
The powertrain control module may be one single computer component, or may contain more than one computer. Chrysler, for example, has several. Because the PCM controls so many functions on an automobile, drivers are usually alerted when a problem occurs by the "Check Engine" light coming on and a mechanic or automotive technician will need to pull the computer error codes to find out where the problem is occurring. Each component a PCM controls is assigned a different code.