Four different criteria have to be met before an engine can start and run.
Since the Powertrain Control Module 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 react, for example, to the loss of signal from the vehicle speed sensor or note that the engine coolant temperature has risen beyond acceptable (programmed) limits.
Once a fault is recognized, a numeric code is assigned and held in memory. The dashboard warning lamp: CHECK ENGINE or SERVICE ENGINE SOON, will illuminate to advise the operator that the system has detected a fault. This lamp is also known as the Malfunction Indicator Lamp (MIL).
More than one code may be stored. Keep in mind not every engine uses every code. Additionally, the same code may carry different meanings relative to each engine or engine family.
In case of a PCM failure, the system will default to a pre-programmed set of values, which allow the engine to operate, although at reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is always affected when the PCM enters this mode.