Chevy vehicles manufactured before 1997 had a diagnostic system called On Board Diagnostics I. This is a diagnostic system monitors all sensors in the vehicle's electrical system and produces codes that help automotive technicians determine where to start when diagnosing problems.
Unfold a paper clip until it is straight then fold it in half.
Remove the driver side kick panel with a screw driver set. There should only be two or three Phillips-head screws.
Locate the ALDL connector. This is a black rectangular 12-prong female connector located close to the center of the dashboard.
Turn off all electrical systems in the vehicle. This includes the heater controls and radio.
Insert the two ends of the paper clip into the A and B terminals of the ALDL connector. These terminals may or may not be labeled, but they will always be on the top right corner of the connector.
Turn the ignition key to the on position without starting the car.
Watch the service engine light. It will initially flash code 12. That means the system is in diagnostic mode. Every code the computer has stored will flash three times, each in sequential order.
