Initial Inspection and Diagnostic Overview

GENERAL DESCRIPTION

The closed loop emission control system has a self diagnostic ability. The ECM constantly monitors the signals from the various sensors, and when a signal deviates from what the ECM "knows" is the normal range, a trouble code is stored in the computer memory. When this happens, the "check engine" light, on the instrument panel, will illuminate. If the fault is a "hard failure" (a problem that is present while vehicle is being serviced) the "check engine" light will remain on while the engine is running. If the failure is intermittent, the light will turn off after a few seconds when the fault is not present, but a trouble code will be stored.
In many cases, a mechanical condition may cause some sensor signals to deviate from their normal range and set codes. For this reason, it is important to make sure that all mechanical systems are in good condition and all engine adjustments are set at the factory specifications before suspecting a computer related failure.

ALDL/DLC Connector:

DISPLAYING TROUBLE CODES

The "Check Engine" light is used to warn the operator of a system failure, and to display any trouble codes that may be stored. When the ignition is turned "on", the "check engine" lamp should lite. This is a bulb check to determine that the lamp is working. When the engine is started, the lamp should turn "off".
A special connector, located in the center console, is used to activate the diagnostic mode. This connector, known as the ALDL (assembly line data link), is also used at the factory to verify that all systems are functioning properly before the vehicle leaves the plant.
To enter diagnostic mode, connect a jumper wire between terminals A (ground) and B (diagnostic test terminal) of the ALDL, with the ignition switch turned on and the engine NOT running. The ECM will display codes by flashing the "check engine" lamp. The first code to display should be a code 12. This is not a trouble code, but indicates that the self diagnostics are functioning. Code 12 is displayed by a flash, followed by a short pause, then two flashes, followed by a longer pause. If other codes are present besides code 12, they will display in the same manner after code 12 is displayed three times. 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. Each code is displayed three times, then the next higher numerical code will be displayed. When all codes have been displayed, code 12 will be repeated. If no trouble codes are stored, code 12 will simply display repeatedly, until the diagnostic mode is exited by removing the jumper wire from the ALDL connector. Do not attempt to start the engine while the diagnostic test terminal is grounded.


CLEARING CODES FROM MEMORY

After displaying and recording any trouble codes, or after repairing any faults in the system, the computer memory should be cleared of all trouble codes by interrupting the power supply to the ECM for 30 seconds. This can be done by removing the ECM fuse (in the fuse block under the dashboard) or by disconnecting the battery. Be sure the ignition is turned off when disconnecting or reconnecting power to the ECM. After clearing trouble codes, it may be necessary to reset digital clocks or radio station presets.


TROUBLE CODE IDENTIFICATION

The following is a list of the trouble codes and the circuits involved.

TROUBLE CODE 13 Oxygen sensor circuit open

TROUBLE CODE 14 Coolant temperature sensor circuit, high temperature indicated (circuit resistance too low, indicates grounded circuit to ECM or shorted sensor)

TROUBLE CODE 15 Coolant temperature sensor circuit, low temperature indicated (circuit resistance too high, indicates open circuit to ECM or open sensor)

TROUBLE CODE 21 Throttle position sensor circuit (signal voltage high)

TROUBLE CODE 22 Throttle position sensor circuit (signal voltage low)

TROUBLE CODE 24 Vehicle speed sensor (VSS) circuit

TROUBLE CODE 32 EGR system failure

TROUBLE CODE 33 Manifold absolute pressure (MAP) sensor circuit (signal voltage high, low vacuum)

TROUBLE CODE 34 Manifold absolute pressure (MAP) sensor circuit (signal voltage low, high vacuum)

TROUBLE CODE 42 Electronic spark timing (EST) circuit

TROUBLE CODE 43 Electronic spark control (ESC) circuit

TROUBLE CODE 44 Oxygen sensor circuit (lean exhaust indicated)

TROUBLECODE 45 Oxygen sensor circuit (rich exhaust indicated)

TROUBLE CODE 51 PROM error (faulty or incorrectly installed PROM or incorrect PROM for application)

TROUBLE CODE 52 CALPAK error (faulty or incorrect CALPAK)

TROUBLE CODE 54 Fuel pump circuit (low voltage)

TROUBLE CODE 55 ECM error