Diagnostic Mode Descriptions

DIAGNOSTIC TEST MODE
When 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 engine control module will:

1. Flash the "Check Engine" malfunction indicator lamp "ON" and "OFF" rapidly (indicating the system is operating correctly).
2. Display any stored trouble codes by flashing the "Check Engine" malfunction indicator lamp. Each data trouble code will be flashed three times, then data trouble code 12 will be flashed again.
3. Energize all engine control module 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 "Check Engine" 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 engine control module will be in the following diagnostic test mode:

- New trouble codes cannot be stored in the engine control module.
- The closed loop timer is bypassed.

LEARNING ABILITY
The Engine Control Module (engine control module) has a "learning" ability which allows it to make corrections for minor variations in the fuel system to improve driveability. If the battery is disconnected to clear diagnostic codes, or for repair, the "learning" process has to begin all over again. A change may be noted in the vehicle's performance. To "teach" the vehicle, make sure the engine is at operating temperature, and drive at part throttle, with moderate acceleration and idle conditions, until normal performance returns.