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. If an intermittent fault has generated a stored trouble code, but does not reoccur within the next 50 engine starts, that code will be erased automatically.
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 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 in the computer memory. 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 three terminal connector, located near the ECM connector, 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 C (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 disconnecting the ECM fusible link wire, 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 - TURBO

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

CODE 12 - No distributor reference pulse (This code does not store in memory, and will only light the check engine lamp when the condition is present with the engine running. If the check engine light comes on intermittently but no code is stored, see DIAGNOSIS BY SYMPTOM under TESTING PROCEDURES.)

CODE 13 - Oxygen sensor circuit (signal from the oxygen sensor is not varying or is absent - this could be caused by an excessively rich or lean condition, or by an oxygen sensor circuit open, shorted to ground, or shorted to power)

CODE 14 - Coolant temperature sensor circuit shorted (abnormal high temperature indicated)

CODE 15 - Coolant temperature sensor circuit open (abnormal low temperature indicated)

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

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

CODE 23 - Manifold air temperature (MAT) sensor (low temperature indicated)

CODE 24 - Vehicle speed sensor (VSS) circuit

CODE 25 - Manifold air temperature (MAT) sensor (high temperature indicated)

CODE 31 - Turbocharger wastegate control

CODE 32 - EGR system fault

CODE 33 - MAP sensor circuit (signal voltage high)

CODE 34 MAP sensor circuit (signal voltage low)

CODE 42 - Electronic spark timing (EST) circuit

CODE 43 - Electronic spark control (ESC) (knock control failure)

CODE 44 - Oxygen sensor circuit (lean exhaust indication)

CODE 45 - Oxygen sensor circuit (rich exhaust indication)

CODE 51 - MEM-CAL (PROM) error (faulty or incorrect PROM)




TROUBLE CODE IDENTIFICATION - NON TURBO

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

CODE 12 - No distributor reference pulse (This code does not store in memory, and will only light the check engine lamp when the condition is present with the engine running. If the check engine light comes on intermittently but no code is stored, see DIAGNOSIS BY SYMPTOM under TESTING PROCEDURES.)

CODE 13 - Oxygen sensor circuit (signal from the oxygen sensor is not varying or is absent - this could be caused by an excessively rich or lean condition, or by an oxygen sensor circuit open, shorted to ground, or shorted to power)

CODE 14 - Coolant temperature sensor circuit shorted (abnormal high temperature indicated)

CODE 15 - Coolant temperature sensor circuit open (abnormal low temperature indicated)

CODE 21 - Idle switch improperly adjusted and/or circuit open (this code will also set if the idle speed falls below 600 rpm for longer than 32 seconds)

CODE 23 - Open or grounded M/C solenoid circuit

CODE 25 - Vacuum switching valve circuit open or grounded

CODE 42 - Fuel cut-off relay and/or circuit shorted

CODE 44 - Lean oxygen sensor indication (circuit voltage low, ECM issuing rich command for longer than two minutes at part throttle under load)

CODE 45 - Rich oxygen sensor indication (circuit voltage high, ECM issuing lean command for longer than two minutes at part throttle under load)

CODE 51 - Faulty calibration unit (PROM) or calibration unit improperly installed

CODE 53 - Shorted switching valve and/or faulty ECM

CODE 54 - Shorted M/C solenoid circuit and/or faulty ECM

CODE 55 - Faulty ECM (problem with analog/digital converter in ECM)