An automobile's powertrain control module (PCM) is its onboard computer. It controls many different functions in a car or truck, including the charging system, emissions and transmission.
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.