Entering Diagnostic Mode

Turn ignition on (engine off). "CHECK ENGINE" light should come on. Locate Assembly Line Data Link (ALDL) connector attached to ECM wiring harness under instrument panel. Insert jumper wire from terminal "A" to "B". See Fig 1

Fig 1: ALDL Connector Terminal Locations
Courtesy of GENERAL MOTORS COMPANY

Insert jumper wire between terminals "A" and "B" when entering diagnostic mode.
"CHECK ENGINE" light should flash code "12" three consecutive times, followed by any trouble codes which may be stored in ECM memory.
If more than one code is stored, they will be displayed from the lowest numbered code to the highest. Each code will flash 3 times. After all codes have been displayed, the complete cycle will repeat. Repetition of cycle continues as long as the "TEST" terminal of ALDL connector is grounded.
CAUTION: When lug is inserted in terminals of ALDL connector, the "TEST" terminal is grounded. DO NOT start engine or turn on ignition while terminal is grounded.

ECM TROUBLE CODE IDENTIFICATION

Trouble Code	Circuit Affected
12	No reference pulse to ECM.
13	Oxygen Sensor Circuit
14	Shorted Coolant Sensor Circuit
15	Open Coolant Sensor Circuit
21	Throttle Position Sensor Circuit
23	Open or grounded M/C Solenoid Circuit
34	Vacuum Sensor Circuit
41	No distributor reference pulses at specific vacuum
42	Electronic Spark Timing circuit or By-Pass grounded or shorted
44	Lean Oxygen Sensor value
44 & 45	Faulty Oxygen Sensor circuit
45	Rich Oxygen Sensor value
51	Faulty PROM or improper PROM installation
54	Shorted M/C or faulty ECM
55	Grounded voltage reference, faulty oxygen sensor or ECM