In 1996, On Board Diagnostics became universal for vehicles sold in the United States. This means the same vehicle diagnostic system is used across all makes and models. The system is often abbreviated as OBD-II. Before this, vehicle manufacturers had their own unique system, and now this first wave of vehicles diagnostics is collectively referred to OBD-I. For Chevrolet vehicles, accessing codes in pre-1996 cars is a relatively easy task. You can jump the system with a paper clip, and the codes will then be flashed at you via the check engine light.
Locate the Assembly Line Data Link outlet in your Chevrolet. It is a 12 pin-receiving hub and it will be located directly beneath the steering wheel.
Connect the "A" and "B" slots with a paper clip. To do this, bend the clip straight, and then bend it into a shape resembling the letter "U." Place an end of the "U" into the "A" slot, and the other into the "B." This slots can be found on the ALDL's upper row, far to the right and at the end. One slot is the last on the row, and the other is next to it.
Leave the Chevrolet's engine off, but turn the key in the ignition to the "On" position.
Record, on paper, the number of times the check engine light flashes at you. Chevrolet's check engine light codes are two character numbers. For example, code 57 will be represented by five long flashes, a pause, and seven shorter flashes. Code 29 will be two long flashes, a pause, a nine shorter flashes. The system will pause for a longer duration between whole codes.
Consult the internet for the Chevrolet's OBD-I code meanings. These codes will not be available in the Chevrolet's owner's manual. However, if you own a Haynes Repair Manual for your vehicle, you will be able to find coding definitions there. Once you have accessed the code meanings, you can make an educated choice on whether or not to take the Chevy to a mechanic or do the repairs yourself.
