Diagnostic Circuit Check
The Diagnostic Circuit Check determines if:
- The "SERVICE ENGINE SOON" light works.
- The ECM is operating and can recognize a fault.
- Any codes are stored.
This is the starting point for any diagnosis. If no codes are indicated, see SYSTEM PERFORMANCE CHECK. If no additional checks are called out from the System Performance Check, see TROUBLE SHOOTING in THEORY & OPERATION article.
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- 1) Check operation of "SERVICE ENGINE SOON" light. With key on, "Scan" tester not connected and engine not running, light should be on steady.
- 2) Grounding test terminal will flash a Code 12 and any stored trouble codes. The light must go on and off to indicate a code. If light goes from bright to dim, see CHART A-6. This is not considered a code.
- 3) If "Scan" tester is not operating properly, try it on another vehicle. If it works on other vehicle, cigar lighter should be checked for battery voltage and a good ground. If "Scan" tester displays "NO DATA" or "NO ALDL" with ignition on, see CHART A6.
- 4) No codes at this point indicate the problem is intermittent and SYSTEM PERFORMANCE CHECK should be performed. If one or more codes are displayed, see appropriate trouble code chart.
- 5) See INTERMITTENTS and SCAN TESTER USAGE in COMPUTER COMMAND CONTROL THEORY & OPERATION section.