Initial Inspection and Diagnostic Overview

ALDL Connector:

CAUTION: To prevent ECM damage, the key must be "OFF" when disconnecting or reconnecting power to the ECM.

READING TROUBLE CODES

The "Check Engine" light on the instrument panel functions to inform the driver that a problem has occurred by remaining "ON" longer that 10 seconds after the engine is started. It is also used by the technician to read trouble codes that assist in system diagnosis.
Provision for reading trouble codes is through the three cavity ALDL connector behind a passenger side kick panel. When cavities "1" and "3" are jumpered with the ignition "ON" and engine "OFF," the ECM enters diagnostic mode and will flash the "Check Engine" light to display any stored trouble codes. Code 12 will flash first followed by any stored codes. If codes are present, the lowest numbered code will flash after code 12, followed by any others in numerical order. Each code will repeat three times until all have been displayed and code 12 is repeated. Code 12 will display repeatedly if no trouble codes are stored.
Trouble codes are read by observing the flashing "Check Engine" light. For example, code 23 is displayed by two flashes at regular intervals, followed by a short pause, then three flashes at regular intervals, followed by a longer pause. The next code is then displayed in a like manner.

CLEARING TROUBLE CODES

Trouble codes will stay in the ECM memory until battery voltage is interrupted. To clear codes, disconnect the negative battery terminal for at least 30 seconds. Codes should be cleared after repairs are completed. After clearing codes by disconnecting the battery, it may be necessary to reset digital clocks, entertainment equipment and other on-board memory data.

TROUBLE CODE DESIGNATIONS

Code 13: Oxygen sensor circuit (open/grounded).
Code 14: Coolant temperature sensor (high or low temperature).
Code 21: Throttle position sensor (open/shorted or low signal voltage).
Code 23: Manifold air temperature (high or low temperature).
Code 24: Vehicle speed sensor.
Code 32: Exhaust gas recirculation (EGR) circuit (California only)
Code 33: Manifold absolute pressure sensor (constant high or low voltage).
Code 42: Electronic spark timing circuit.
Code 44: Oxygen sensor circuit (lean exhaust).
Code 45: Oxygen sensor circuit (rich exhaust).
Code 51: ECM failure.