Diagnostic Circuit Check

The diagnostic circuit check makes sure that the self-diagnostic system works, determines that the trouble codes will display, and guides diagnosis to other problem areas. When the engine is running and a problem develops in the system which the self-diagnosis can evaluate, the "CHECK ENGINE" light will come "ON" and a trouble code will be stored in the ECM Trouble Code Memory." The light will remain "ON", with the engine running as long as there is a problem. If the problem is intermittent, the "CHECK ENGINE" lamp will go out, but the trouble codes will be stored in the ECM trouble code memory.

With the ignition turned "ON" and the engine stopped, the "CHECK ENGINE" light should be "ON". This is a bulb check to indicate that the lamp is working.

A three (3) terminal connector, located near the ECM connector, is used to access the trouble code system in Line Communications Link. Two terminals of this connector ("A" and "C") are used to activate the trouble code system.

With the ignition "ON", a jumper wire connected between terminal "A" and "C", will enable the trouble code system.

The "CHECK ENGINE" light will begin to flash a Trouble Code "12". Code 12 consists of one flash, a short pause, then two flashes. There will be a linger pause and Code 12 will repeat two more times. This check indicates that the self-diagnostic system is working. After Code 12 has flashed three times, any other trouble codes that have been stored in memory will be flashed out. Each stored code will flash three times. A final Code 12 will be flashed to indicate all stored codes have been displayed. The cycle will then repeat itself until the engine is started or the ignition is turned "OFF". Remove the ground from test terminal before starting engine.

A trouble code indicates a problem in a given circuit (i.e., trouble Code 15 indicates a problem in the coolant sensor circuit). This includes the coolant sensor, connector, harness, and Electronic Control Module (ECM). The procedure for finding the problem can be found in the the appropriate "Diagnosis Trouble Code Chart". Charts are provided for each trouble code. Testing and Inspection

When the engine is started, the "CHECK ENGINE," light will go "OFF". If the "CHECK ENGINE" light remains "ON", the self-diagnosis system has detected a fault.

If a trouble code is obtained when the "CHECK ENGINE" light is "OFF" with the engine running, the trouble code must be evaluated. A determination must be made to see if the fault is intermittent or if the engine must be at certain operating conditions to turn the "CHECK ENGINE" light "ON".

Faults indicated by trouble Codes 13, 44 and 45 require engine operation at part throttle for up to five minutes before the "CHECK ENGINE" light will come "ON".

The diagnostic charts for trouble codes 13, 44 and 45 should be used if any of these trouble codes are obtained.