This has been explained many times and the question still comes up. Hope this helps.
Codes will vary so once you get a code and still need help get back to us with as much info as you can (engine, year, fuel system, engine vin code, etc.)
FAQ: How to read older GM codes for free
If you own a 1982 to 1995 (starting on some 79 vehicle in California) 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.
The ALDL should be under the dash to the right of the column)
This is referred to as the Assembly Line Communication Link (ALDL) connector.
How to read the codes:
With a paper clip, connect terminals A and B together (see diagram below). Turn 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)