With Diagnostic Scan Tool

Diagnostic Circuit Check:

SYSTEM CHECK
Since this is the starting point for the diagnostic procedure or finding the cause of an emissions test failure, always begin here. The system check is performed through the twelve terminal Assembly Line Diagnostic Link (ALDL) connector under the instrument panel in the passenger compartment.
The Computer Command Control system check is a procedure that determines the following:

1. Bulb Check, checks the Service Engine Soon (SES) light circuit and that the computer can complete the circuit. The system check starts with a bulb check. If there is no "SERVICE ENGINE SOON" light, refer to Diagnostic Charts / Chart A-1, No "Service Engine Soon" Light.

2. Diagnostic Mode, this indicates if the diagnostic mode is working.

3. Does "Scan" Display, this determines if the computer is supplying input and output visual data.

4. Does engine start, this step is done after it has been determined that the computer will display codes and data.

5. Any codes displayed, proceed to the applicable chart if a code is displayed. Scanning the data for typical values may indicate a problem area if the values are different from typical, refer to Typical Scan Data Values.
If all systems appear to be functioning, refer to Diagnosis By Symptom.

NOTE: Not all "Scan" testers are compatible with every system. A Scan tool that produces faulty readings should NOT be used, and the problem should be reported to the tool manufacturer. Use of a faulty Scan tool can result in improper diagnosis and unnecessary component replacement. Follow the proper instructions, located in the owners manual, for the specific Scan tool being used.