On-Board Diagnostics

If a problem develops in a monitored system with the engine running, the "Check Engine" light will come "ON" and a trouble code 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 trouble code will be stored in ECM memory. As a bulb check, the "Check Engine" light 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.

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

ACCESSING TROUBLE CODES

ALDL Connector:

To access trouble codes, a three terminal connector known as the ALDL (Assembly Line Diagnostic Link) is utilized. Connecting terminals "A" and "C" with a jumper wire and turning the ignition "ON" activates the ECM trouble code system. The "Check Engine" light 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 Trouble Codes:

If trouble codes 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. Code 14, for example, indicates a problem in the coolant sensor circuit.

Faults indicated by codes 13, 44 and 45 require engine operation at part throttle for up to five minutes after engine warm-up before the "Check Engine" light will come on and store a code. Code 15 takes five minutes of engine operation before it will display. Remove the jumper wire before starting the engine.

CLEARING CODES FROM MEMORY

CAUTION: To avoid component damage, be sure ignition is "OFF" when disconnecting or reconnecting power to the ECM.

The trouble code memory receives a continuous 12 volts even with the ignition "OFF." After displaying and recording any trouble codes or after making repairs, the ECM memory should be cleared by interrupting the power for at least 10 seconds. This is accomplished by temporarily removing the red ECM fusible link wire at the battery or by disconnecting the battery. 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 may come 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.
6. Arcing at ignition coil, plug wires or spark plugs.
7. Poor power supply circuit connections.
8. "Check Engine" light wire to ECM shorted to ground.
9. Diagnostic test lead to ECM shorted to ground.
10. Poor ECM ground connections at either terminal "1" or "20."
11. 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.
12. Improper installation of accessories such as lights, sound systems, alarms, etc.


TROUBLE CODE IDENTIFICATION

CODE 12 No ignition reference pulses to ECM. This code is not stored in memory and will only flash while the fault is present. It is displayed before the engine is started.

CODE 13 Oxygen sensor circuit. The engine must run up to two minutes at part throttle, under road load, before this code will set.

CODE 14 Shorted coolant sensor circuit. The engine must run up to two minutes before this code will set.

CODE 15 Open coolant sensor circuit. The engine must run up to five minutes before this code will set.

CODE 21 Idle switch/throttle position sensor faulty, improperly adjusted and/or circuit open. This code will also set if engine speed falls below 600 rpm for longer than 32 seconds.

CODE 22 Fuel cut relay faulty and/or circuit open.

CODE 23 Duty solenoid circuit open or grounded.

CODE 25 Vacuum switching valve circuit open or grounded.

CODE 44 Lean oxygen sensor indication. The engine must run for at least two minutes at part throttle and road load before this code will set.

This code will not set under any of the following conditions:
1. Coolant temperature below 70°C (158°F).
2. Air cleaner air temperature below 0°C (32°F)-"low altitude" condition.
3. "High altitude" condition.

CODE 45 Rich system indication. The engine must run for at least two minutes at part throttle and road load before this code will set.

This code will not set under any of the following conditions:
1. Coolant temperature below 70°C (158°F).
2. Engine speed below 1500 rpm or above 2500 rpm.
3. "High altitude" condition.


CODE 51 Faulty PROM or incorrect installation. It takes up to 30 seconds before this code will set.

CODE 53 Shorted VSV and/or faulty ECM.

CODE 54 Shorted duty solenoid circuit and/or faulty ECM.

CODE 55 Faulty A/D converter in ECM.