Reading Diagnostic Trouble Codes

If a problem develops in a monitored system with the engine running, the MIL will come "ON" and a DTC will be stored in ECM memory. As long as a problem is present, the light will remain "ON" while the engine is running. If the problem is intermittent, the light will go out but a DTC will be stored in ECM memory. As a bulb check, the MIL will come "ON" with the ignition switched "ON" and the engine not running. If no codes are present, the light will go out upon starting the engine. If the light remains "ON," the system has detected a fault.

ACCESSING DTCs

Malfunction Indicator Light (MIL):

Diagnostic Connectors:

To access DTCs, a test lead (white wire) and ground lead (black wire) are branched from the wiring harness near the ECM. With the ignition "ON" connect these two leads together. The MIL will begin to flash code 12. Code 12 consists of one flash, a short pause, then two flashes. After a longer pause, code 12 will repeat two more times. This check indicates that the on-board diagnostic system is functioning. If no other codes are present, the cycle will repeat until the ignition is switched "OFF."

Reading DTCs:

If DTCs are stored in memory, the lowest number code will flash three times followed by the next higher code number until all stored codes have been displayed. The codes will then repeat in the same order until the test lead is disconnected.

INTERMITTENT MIL

"Intermittent" means that the MIL comes on at times but does not stay on. Since codes may or may not be stored, the use of diagnostic charts could result in replacement of good parts.

Most intermittent problems are caused by faulty electrical wiring or connections. Carefully check these potential causes:

1. Poor mating of connector halves or terminals not fully seated in connector body.
2. Deformed or damaged terminals. All connector terminals in problem circuit should be carefully reformed to increase contact tension.
3. Poor terminal-to-wire connection. This requires removing terminal from connector body to properly check.
4. Poor ground connections.
5. If a visual check does not find the source of the problem, the vehicle may be driven with a voltmeter connected to a suspect circuit. An abnormal voltage reading when the problem occurs indicates the problem may be in that circuit.

An intermittent MIL with no stored code may be caused by:

1. Arcing at ignition coil, plug wires or spark plugs.
2. Faulty power supply main relay.
3. Poor power supply circuit connections.
4. MIL wire to ECM shorted to ground (circuit 22).
5. Diagnostic test lead to ECM shorted to ground (circuit 24).
6. Poor ECM ground connections at intake manifold.
7. Electrical system interference caused by a defective relay, ECM driven solenoid or switch. These problems cause an electrical surge and normally occur when the faulty component is operated.
8. Improper installation of accessories such as lights, sound systems, alarms, etc.