Description of On-Board Diagnostics
DIAGNOSTIC TEST MODEWhen the Diagnostic Test Terminal is grounded with the Ignition "ON" and the Engine "OFF", the system will enter what is called the Diagnostic Test Mode. In this diagnostic test mode the ECM will:
1. Flash the MIL "ON" and "OFF" rapidly (indicating the system is operating correctly).
2. Display any stored trouble codes by flashing the MIL. Each DTC will be flashed three times, then DTC 12 will be flashed again.
3. Energize all ECM 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.
4. The Idle Air Control Valve moves to its fully extended position on most models, blocking the Idle Air Passage. This is useful in checking the minimum idle speed.
FIELD SERVICE MODE
If the Diagnostic Test Terminal is grounded with the Engine running, the system is in "Open" or "Closed Loop".
In "Open Loop" the Malfunction Indicator Lamp flashes two and one-half times per seconds. In "Closed Loop", the light flashes once per second.
Also, in "Closed Loop", the light will stay "OFF" most of the time if the system is too lean. It will stay "ON" most of the time if the system is too rich. While the system is in Field Service Mode, the ECM will be in the following diagnostic test mode:
1. New trouble codes cannot be stored in the ECM.
2. The closed loop timer is bypassed.