See Figures 1, 2 and 3
The Powertrain Control Module (PCM) performs a continual self-diagnosis on many circuits of the engine control system and, if equipped, on the automatic transaxle control system. If a problem or irregularity is detected, the PCM will set either a code or a flag. A code indicates a suspected failure that currently exists or that has existed in an engine system. A currently existing code will illuminate the SERVICE ENGINE SOON light. A flag is a diagnostic aid that indicates an intermittent problem or irregularity, but is does not necessarily indicate a failure. A flag will not normally illuminate the indicator light.
There are 2 levels of PCM memory, General Information and Malfunction History. Codes are stored in both places, while flags are only stored in malfunction history. Any code stored in General Information can be read by observing the flashing SERVICE ENGINE SOON lamp while the system is in diagnostic mode. Flags and codes that are stored in Malfunction History can only be retrieved with a suitable scan tool. Most tools will be menu-driven and, although their use is not specifically covered in this section, most described tests are useful for interpreting scan tool test results. Unlike flags for the engine control system, flags which are designated for the automatic transaxle will flash on the SHIFT TO D2 lamp (1991-1992 models) or HOT lamp (1993-98 models), along with any stored transaxle codes.