Self-Diagnostic System: Notes

NOTE: For On-Board Diagnostic System Check see the F - BASIC TESTING - 7.4L article.

NOTE: Powertrain Control Module (PCM) may also be referred to as Vehicle Control Module (VCM) in some diagnostic text and illustrations. Terms may be used interchangeably.

PCM/VCM is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, PCM/VCM will store a Diagnostic Trouble Code (DTC) and, in most cases, illuminate the Malfunction Indicator Light (MIL) located on instrument cluster. Malfunctions are recorded as hard failures or as intermittent failures.

There are 4 types of DTC category:

Type "A" - Emissions related, turns on MIL the first time DTC sets.
Type "B" - Emissions related, turns on MIL if fault is active for 2 consecutive driving cycles.
Type "C" - Non-emissions related, does not turn on MIL, but will turn on SERVICE light.
Type "D" - Non-emissions related, does not turn on MIL or SERVICE light.