I had a '90 Chevy pickup that was OBDI. I have a little box that plus into the diagnostic port and causes the CEL to flash out codes which you can look up in this fat little book that came with the checker.

Before OBDII, manufacturers had a variety of ways to pull codes from the systems such as the aforementioned "paperclip" method which worked on some early Toyotas and others.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250