Clearing Diagnostic Trouble Codes

The closed loop emission control system has a self diagnostic ability. The ECM constantly monitors the signals from the various sensors, and when a signal deviates from what the ECM programmed normal range, a trouble code is stored in the computer memory. When this happens, the "Check Engine" light will illuminate. If the fault is a "hard failure" (a problem that is present while vehicle is being serviced) the "Check Engine" light will remain on while the engine is running. If the failure is intermittent, the light will turn off after a few seconds when the fault is not present, but a trouble code will be stored.

In many cases, a mechanical condition may cause some sensor signals to deviate from their normal range and set codes. For this reason, it is important to make sure that all mechanical systems are in good condition and all engine adjustments are set at the factory specifications before suspecting a computer related failure.

Refer to DIAGNOSTIC CIRCUIT CHECK for information on checking the function of the on-board diagnostic system.


ACCESSING TROUBLE CODES

Diagnostic Lead And "Check Engine" Light:

Connect together test terminals near the ECM. This will cause the light to flash code 12, indicating that "Diagnostic Mode" is working. Code 12 will flash three times, followed by any other codes stored in ECM memory. Each code will flash three times starting with the lowest numbered code and then repeat starting with code 12. If there are no other codes, code 12 will flash repeatedly until the test terminals are disconnected or the engine is started.


CLEARING CODES FROM MEMORY

ECM Fuse:

With ignition "OFF," remove fuse #4 (7.5A) in the fuse panel on the left kick panel or disconnect the battery for at least 10 seconds. Codes will also be cleared when the 13 pole connector is disconnected from the ECM. After clearing codes, it may be necessary to reset electronic accessories such as digital clocks and radios.

INTERMITTENT "CHECK ENGINE" LIGHT

"Intermittent" means that the "Check Engine" light 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 "Check Engine" light 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. "Check Engine" light wire to ECM shorted to ground (circuit 21).
5. Diagnostic test terminal wire to ECM shorted to ground (circuit 44).
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.

TROUBLE CODE IDENTIFICATION

CODE 13 Oxygen sensor open circuit or sensor deterioration.

CODE 14 Coolant temperature circuit shorted.

CODE 15 Coolant temperature circuit open.

CODE 21 Throttle valve switch system. Contacts for both idle and full throttle making contact simultaneously.

CODE 22 Starter signal system does not send signal to ECM.

CODE 23 Ignition system power transistor output terminal shorted.

CODE 25 Pressure regulator vacuum switching valve circuit shorted or open.

CODE 26 Canister purge vacuum switching valve circuit shorted or open.

CODE 27 Canister purge vacuum switching valve circuit. Defective transistor or grounding system in ECM.

CODE 32 EGR system fault.

CODE 33 Fuel injection system. Output terminal shorted or open circuit.

CODE 34 EGR temperature sensor or harness defective.

CODE 35 Ignition system power transistor system open circuit.

CODE 41 Crank angle sensor system. No signal to ECM.

CODE 43 Throttle valve switch system. Idle contact "ON" continuously (code does not set when air flow sensor system is defective).

CODE 44 Oxygen sensor lean signal (low voltage).

CODE 45 Oxygen sensor rich signal (high voltage).

CODE 51 ECM ROM faulty.

CODE 52 ECM RAM faulty.

CODE 53 Pressure regulator vacuum switching valve circuit. Defective transistor or grounding system in ECM.

CODE 54 Ignition system power transistor system. Defective transistor or grounding system in ECM.

CODE 61 Air flow sensor system. Open or shorted circuit or broken hot wire.

CODE 62 Air flow sensor system. Broken cold wire.

CODE 63 Vehicle speed sensor. No signal to ECM.

CODE 64 Fuel injection system. Defective transistor or grounding system in ECM.

CODE 65 Throttle valve switch system. WOT contact "ON" continuously.

CODE 66 Knock sensor system open or grounded circuit.

CODE 71 Throttle position sensor system (Turbo). Abnormal signal to ECM.

CODE 72 EGR vacuum switching valve system circuit open or shorted.

CODE 73 EGR vacuum switching valve system. Defective transistor or grounding system in ECM.