obd1 and 2 you can jump the serv conn in the pass footwell. long blink=10, short blink=1. there is a long pause between codes. example: 1 long and 3 short=code 13. look in a haynes/chiltons/helms and see what that code is, or look up on the internet.

if you're obd2 and too lazy to do it yourself they can scan it from the data port and tell you what's wrong.