Reading Diagnostic Trouble Codes

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.

If the diagnostic terminal is grounded with the engine running, the system enters Field Service mode and the "Check Engine" light will indicate whether it is in "Open Loop" or "Closed Loop." In "Open Loop," the light flashes two and one half times per second. In "Closed Loop," the light flashes once per second. If the system is running lean while in "Closed Loop," the light will stay "OFF" most of the time. Conversely, it will stay "ON" most of the time if the system is running rich. While in Field Service mode, the ECM will not store new trouble codes.

Note that many electronic engine control tests require the use of a "Scan" tool.

ACCESSING TROUBLE CODES

ALDL/DLC Connector:

The "Check Engine" light displays trouble codes that are stored in ECM memory. To activate the Diagnostic mode, a special connector located in the center console is utilized. When the ALDL (assembly line diagnostic link) connector is jumpered between terminals "A" and "C" with the ignition "ON" and the engine not running, the light will flash any stored codes.

The first code to display should be code 12. This indicates that the self-diagnostics are functioning and is not a trouble code. The code 12 display is a flash followed by a short pause, then two flashes followed by a longer pause. If other codes are present, they will display in the same manner after code 12 has flashed three times. For example, the code 23 display is two flashes followed by a short pause, then three flashes followed by a longer pause. Each stored code is displayed three times, starting with the lowest in numerical order. Code 12 will repeat when all codes have shown. If no codes are stored, code 12 will display repeatedly until Diagnostic mode is exited by removal of the ALDL jumper wire. Remove the jumper wire before attempting to start the engine.

To allow checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions, all ECM controlled relays and solenoids (except fuel pump relay) are energized in Diagnostic mode.


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. If wiring and connectors check okay and a code was stored for a circuit having a sensor (except codes 44 and 45), substitute a known good sensor and recheck.
6. Loss of ECM trouble code memory. To check, disconnect throttle position sensor and idle engine until "Check Engine" light comes "ON." Code 22 should be stored in memory for at least 10 seconds after ignition is turned "OFF." If not, the ECM is faulty.

An intermittent "Check Engine" light with no stored code may be caused by:

1. Arcing at ignition coil, plug wires or spark plugs.
2. EST wires should be routed away from spark plug wires, ignition system components and alternator. ECM to ignition system circuit 453 must have a good ground connection.
3. Poor power supply circuit connections.
4. "Check Engine" light wire to ECM shorted to ground (circuit 419).
5. Diagnostic test lead to ECM shorted to ground (circuit 451).
6. Poor ECM ground connections.
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.