FAQ: How to read older GM codes for free

If you own a 1982 to 1995 GM car or truck, you just might be able to read your trouble codes for free! If you have a connector like this under your dash, read on... (note: this connector may have a black plastic cover over it, so also look for a cover shaped like this, sorry for the messy wires here)

This is referred to as the Assembly Line Communication Link (ALDL) connector.

How to read the codes

• Ground terminals A and B (see diagram below) with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a Code 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A Code 12 consists of one flash, a short pause, then two flashes in rapid succession.
  
• After Code 12 is displayed, any stored trouble codes will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All codes are displayed 3 times each then a code 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

ALDL Diagnostics Connector (Located Underdash)