My 1991 Trans Am Convertible quit running after the "Service Engine Soon" light came on. My Haynes manual says to check it for codes, but doesn't tell me where it is. Where is it?

 

IIRC the port to plug in the code reader is under the dash, above where your right knee would be. I had a 1987 IROC-Z, and purchased an OBD-1 reader for about $40 (this was a long time ago). Plugging it in would cause the "Service Engine Soon" light to flash. Count the flashes, look up the number in the book, find your code. The flashes cycle through, so if you have multiple codes, they just repeat. Some owners skip the reader, and use a paperclip to connect terminals, but I wasn't interested in doing that.

 

It has been a while and I'm going by memory. You can use a paperclip to ground the last two ports on the farthest right side of the connector. Turn ignition key to run then the check engine light will flash 12 three times, then it'll flash you the trouble code 3 times and it will keep this repeating pattern until you turn the key off. Try that and see if it works.