Diagnostic Circuit Check
The diagnostic circuit check is an organized approach for identifying a problem caused by an electronic control system malfunction. If after completing check, no problems were found, a comparison of "Scan" tester parameters may be used to help locate intermittent and out-of-specification sensors. See SCAN TESTER - TEST DATA PARAMETERS table in this article.
If the "Scan" tester is not operating properly, check to verify it's operation on another vehicle. If okay, the cigar lighter socket should be checked for 12 volts and a good ground. If the "Scan" tester reads "NO DATA" or "NO ALDL", with the ignition on, check the serial data wire for an open or short to ground between ALDL terminal "M" and the ECM. Also check for an open diagnostic test terminal wire from ALDL terminal "B" to ECM. Use mini schematic from CHART A1 - NO "SERVICE ENGINE SOON" ("SES") LIGHT for terminal reference.