The Diagnostic Executive is a unique segment of software in the PCM designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are listed below:
A key function of the Diagnostic Executive is to record codes and turn on the MIL when an emission related fault is detected. It can also turn off the MIL if the Conditions: go away that caused the code to set.
The PCM continually tests the operation of certain engine control functions. This diagnostic capability is complimented by the diagnostic procedures contained in repair manuals. The language of communicating the source of the fault detected occurs through a set of trouble codes. When a fault is detected by the PCM, a code is set and the MIL is illuminated for all emission related trouble codes.