How to Read GM OBD-I Codes with a Paperclip

What this covers

GM built the same flash-code system into almost every car and light truck from 1982 to 1995. The computer stores a code, then blinks it out on the check-engine light when you ask. You need a paperclip or a short jumper wire, and that is all. No scanner, no app, no subscription.

Find the ALDL connector

The connector is the ALDL, the Assembly Line Diagnostic Link. It is a rectangular socket, usually under the dash on the driver side. On some models it hides behind a small panel under the steering column. Look for a row of lettered terminals. A and B are the two you bridge.

Bridge terminals A and B

Key off. Bridge A and B on the ALDL connector with the paperclip. Turn the key to run with the engine off. The check-engine light starts flashing. Do not start the engine yet.

Read the flashes

Every fault code flashes three times so you get three passes at it. A two-digit code is a group of flashes, a pause, then a second group. Code 24 is two flashes, pause, four flashes. Write each number down as it blinks. The light runs through the stored codes in order, then repeats.

Code 12 is not a fault

Code 12 flashes first on almost every GM car. It means the computer is in diagnostic mode and ready to talk. On a running car it can also mean no distributor reference pulse, but with the engine off it is just the ready signal. Do not replace parts for code 12 alone.

Clear the code and recheck

Fix the fault first. Then clear the code by pulling the ECM fuse or disconnecting the battery for a short spell. Drive the car like normal and check again. If the light and the code come back, the fault is still there. A code points at a circuit, not at a part.

Look up your number

Find the code in the GM OBD-I code list. Each page gives the meaning, the usual causes, and the order to check them. Start at the top of the check list and work down before you buy anything.