Ensure ignition switch is in the OFF position. Using a jumper wire, jumper Assembly Line Diagnostic Link (ALDL) connector diagnostic test terminals No. 1 and No. 3. See Fig 1 . Connector is located behind kick panel in the passenger compartment. Turn ignition switch to ON position. Read trouble code(s). Proceed to CODE CHARTS for diagnosing stored codes.
If no trouble codes are stored, CHECK ENGINE light will flash rapidly on then off until jumper is disconnected. Count the number of flashes and look for pauses in flashing to ensure no trouble codes are overlooked. If trouble codes are stored, code will flash 3 times and the lowest code will be given first.

Fig 1: Identifying ALDL Connector

Courtesy of GENERAL MOTORS CORP.

TROUBLE CODE IDENTIFICATION CHART

Code	Circuit Affected	Probable Cause
13	Oxygen Sensor	Open/Grounded Circuit
4A	Coolant Temp. Sensor	High Temp. Indicated
14B	Coolant Temp. Sensor	Low Temp. Indicated
21A	Throttle Position Sensor	Signal Voltage High
21B	Throttle Position Sensor	Signal Voltage Low
23A	Manifold Air Temp. Sensor	Low Temp. Indicated
23B	Manifold Air Temp. Sensor	High Temp. Indicated
24	Vehicle Speed Sensor	Incorrect Signal Voltage
33A	MAP Sensor	Signal Voltage High Low Vacuum
33B	MAP Sensor	Signal Voltage Low High Vacuum
42	Electronic Spark Timing	Open/Shorted Circuit
44	Oxygen Sensor	Lean Exhaust Indicated
45	Oxygen Sensor	Rich Exhaust Indicated
51	ECM Failure	ECM Failed Or EE PROM Failure

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Retrieving Codes