How to Read GM OBD-1 DTC's

     Reading codes in any GM OBD-1 (pre-1996) vehicle is really simple to do at home, with no special tools.  Unless you consider an ordinary paper clip a special tool.  Don't be lured into purchasing a fancy "code reader" for these vehicles!

1.  The first thing you need to do (after procuring a paper clip) is to locate the vehicle's ALDL (assembly line data link) usually found under the driver's side of the dash (red arrow below).  The example below is typical of 1988-1994 Chevrolet/GMC trucks. 

2.  Locate ports "A" and "B" in the ALDL connector.  These will be the the two ports in the upper right hand corner.

3.  Make sure that the ignition key is off, and form your paper clip into a "U" shape. 

3.  Insert the ends of the paper clip into ports "A" and "B" of the ALDL connector.

4.  Turn the key to the "On" position, but DO NOT crank the engine!  Damage may result.  The check engine light will now begin a series of flashes.  It should start with a code "12" (diagnostic test active).  It will flash this code three times.  "12" will be a single flash, followed by a series of two flashes.  Then a pause, followed by a single flash, then a series of two flashes.  Then another pause, a single flash, and a series of two flashes.

Now, if any DTC's are present, they will begin to flash in the same manner.  For example, code "32" (EGR system failure) will be represented by a series of three flashes, then a series of two flashes.  Each code will repeat three times, and code "12" will repeat once all codes have been displayed.

This sequence will continue flashing until the key is returned to the off position.  Don't forget to remove your paper clip from the diagnostic port when you are finished!