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 ALDL connector usually located in the center console and sometimes covered by a plastic cover labeled "DIAGNOSTIC CONNECTOR." When ALDL terminals "A" and "B" 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.
Depending on how the vehicle is equipped, the ECM power feed may also be disconnected at the positive battery terminal "pigtail," the inline fuse holder originating at the positive battery connection or the ECM fuse in the fuse panel.

TROUBLE CODE DESIGNATIONS

Code 13: Oxygen sensor circuit (open).
Code 14: Coolant temperature sensor (high temperature).
Code 15: Coolant temperature sensor (low temperature).
Code 21: Throttle position sensor (signal voltage high).
Code 22: Throttle position sensor (signal voltage low).
Code 24: Vehicle speed sensor.
Code 32: EGR system failure.
Code 33: Manifold absolute pressure sensor (high voltage/low vacuum).
Code 34: Manifold absolute pressure sensor (low voltage/high vacuum).
Code 42: Electronic spark timing circuit.
Code 43: Electronic spark control circuit.
Code 44: Oxygen sensor circuit (lean exhaust).
Code 45: Oxygen sensor circuit (rich exhaust).
Code 51: PROM error.
Code 52: Calpak error.
Code 54: Fuel pump circuit (low voltage).
Code 55: ECM error.