Diagnostic Mode Description

When the diagnostic terminal is grounded with the ignition "ON" and the engine "OFF," the system will enter what is called the Diagnostic Mode. In this mode the Powertrain Control Module (PCM) will:

1. On certain vehicles with a 12 pin Data Link Connector (DLC) (pin "B" included), display a Diagnostic trouble code (DTC) 12 by flashing the MIL (Service Engine Soon) (indicating the system is operating correctly). If any additional DTC's are set, it will flash those DTC's. Each DTC will be flashed three times, then DTC 12 will be flashed again.
2. Energize most system controlled relays and solenoids except fuel pump relay. This allows checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions.
3. The Idle Air Control (IAC) valve moves to its fully extended position on most models, blocking the idle air passage. This is useful in checking the minimum idle speed.