GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Reading Codes

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The diagnostic trouble codes can be read by grounding test terminal B. Terminal A is the internal PCM ground. Using a jumper from B to A will read the stored codes. These are the last two terminals on the upper right of the ALDL connector.

Once the terminals have been jumped, turn the ignition switch on, leave the engine off .

The MIL light should be flashing. If it is not, turn the ignition switch OFF and remove the jumper. Turn the ignition on and confirm that the light is now on. If it is not, replace the bulb and try again.

The codes stored in memory may be read through counting the flashes of the dashboard-warning lamp. The dash-warning lamp should begin to flash Code 12. The code will display as one flash, a pause and two flashes. Code 12 is not a fault. It is used as a system acknowledgment or handshake code; its presence indicates that the PCM can communicate as requested. Code 12 used to begin every diagnostic sequence. Some vehicles also use Code 12 after all diagnostic codes have been sent.

After Code 12 has been transmitted 3 times, the fault codes, if any will each be transmitted 3 times. The codes are stored and transmitted in numeric order from lowest to highest.

The order of the codes in memory does not indicate the order of occurrence.

 
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