Description of On-Board Diagnostics

CIRCUIT OPERATION
The ECM will substitute the input signals for the air flow meter, engine temperature sensor, intake air temperature sensor and the throttle sensor if the circuits fail. This will allow the vehicle to driven until repairs can be made.

When in fail-safe, the vehicle will run but engine speed will be limited to 2400 rpm. The CHECK ENGINE light will also come on to alert the driver of the problem.

Air Flow Meter Fail-Safe Operation:

MASS AIR FLOW SENSOR
If the ECM senses that the mass air flow sensor output voltage is out of the normal range it will substitute the throttle sensor signal for the correct signal. Engine rpm will be limited to 2400 in order to inform the driver that the ECM is in the "Fail-Safe Mode". Refer to Image for operating conditions.

Temperature Sensor Fail-Safe Operation:

ENGINE TEMPERATURE SENSOR MALFUNCTION
When the ECM senses that the engine temperature output voltage is out of the normal range, it will substitute voltage at preset values. Refer to image for operating conditions.

Throttle Sensor Fail-Safe Operation:

THROTTLE SENSOR MALFUNCTION
When the ECM senses that the throttle sensor voltage is out of the normal range it will substitute a voltage signal at a preset value. Refer to image for operating conditions.