GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Entering Self-Diagnosis

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To enter the diagnostics, either connect a scan tool to the ALDL/DLC connector (carefully following the tool manufacturer's instructions) or use a jumper wire to connect terminals A and B and begin flash diagnosis. On carbureted vehicles, the jumper should be connected with the ignition switch turned ON to begin code display. For fuel injected vehicles, the jumper should be connected with the ignition switch OFF , then with the jumpers in place, the switch should be turned ON to begin code read out. In both cases, the engine should NOT be\ running. Once the jumper is properly connected, the control module will enter the diagnostic program and report trouble codes on the scan tool or by flashing the MIL.

All Codes are 2 digits between 12 and 82. Codes are displayed on the CHECK ENGINE or SERVICE ENGINE SOON light by flashing the MIL with short and long pauses to distinguish digits of 1 code from digits of another. The short pause is used between digits of the same code, long pauses are between codes. For Code 12, the sequence will be: flash, pause, flash-flash, long pause.

  1. When the diagnostic mode is entered, Code 12 is displayed three times. This indicates that the internal diagnostic system is operating. If Code 12 is not displayed, the self-diagnostic program is not functioning properly. If no system malfunctions have been stored, Code 12 will continue to repeat.
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On many applications, entering diagnostic mode will cause solenoids and relays to activate. Pulse-width modulated solenoids may be rapidly energized and de-energized, causing a clicking noise. Do not be startled by or concerned with this underhood noise during self-diagnosis code display.

  1. After code 12 flashes to indicate diagnostic display, any existing system fault codes are displayed in order from lowest to highest code number. Each code is displayed 3 times, followed by the next code, if any.
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  3. When all codes have been displayed, Code 12 will flash again.
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