Self-Diagnostic System: Notes

WARNING: This page is about a different car, the 1997 Pontiac Grand Am, 1997 Oldsmobile Cutlass, 1997 Oldsmobile Achieva, 1997 Chevrolet Malibu, and 1997 Buick Skylark. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: For On-Board Diagnostic System Check see the BASIC TESTING - 3.1L 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 categories. See the following list:

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.