A Power Train Control Module (PCM) is the heart of this control system. A network of sensors provides the PCM with information about engine operation and the various systems that it controls. The on-board diagnostic system check verifies the system is functioning correctly. The PCM has the ability to do some diagnosis of itself, as well as other parts of the system. When the PCM finds a problem, it lights a Malfunction Indicator Light (MIL) on the instrument panel (Check Engine or Service Engine Soon) and a trouble code is stored in the PCM memory. The MIL does not indicate that the engine should be stopped right away, but that the vehicle should be checked as soon as reasonably possible.
The Malfunction Indicator Lamp (MIL) will illuminate when the key is turned to the ON position as a systems check, but will extinguish shortly after the engine is started. If the computer control module detects a malfunction in one of the monitored circuits, a trouble code will be set and the lamp will be illuminated to indicate a fault.
When the computer module sets a code, the MIL will remain illuminated as long as the fault is detected. If the problem is intermittent, the light will extinguish approximately ten seconds after the fault goes away. However, the code will stay in the memory for fifty starts or until the code is cleared.
Obviously, the MIL cannot be reset until the malfunction is corrected (or at least not present if it is an intermittent fault) and the computer memory is cleared of the fault. Codes may be cleared using a suitable scan tool or by removing power from the computer control module. If the latter method is chosen power must be removed from the module for a minimum for thirty seconds. The lower the ambient temperature, the longer period of time the power must be removed.