SELF-DIAGNOSTICS
PCM/TCM constantly monitors all electrical circuits. If PCM/TCM detects circuit problem(s) or out-of-range sensor(s), a Diagnostic Trouble Code (DTC) will be recorded in computer memory. If problem continues for a preset time, the Malfunction Indicator Light (MIL) or SERVICE ENGINE SOON light will glow.
If the MIL (SERVICE ENGINE SOON) light is always on, DTC is currently being detected. If the MIL (SERVICE ENGINE SOON) light is off, but PCM/TCM has detected a circuit or sensor problem, DTC's will be stored in computer memory.
Stored DTC's may be retrieved from PCM/TCM memory using scan tool or MIL (SERVICE ENGINE SOON) light. See appropriate RETRIEVING CODES procedure under ELECTRONIC SELF-DIAGNOSTICS.
Note. For complete PCM/TCM testing and diagnosis, see appropriate article in ENGINE PERFORMANCE of appropriate manual.