"Non-Scan" Version
The diagnostic circuit check is an organized approach for identifying a problem caused by an electronic control system malfunction. If after completing the diagnostic circuit check, no problems were found, a comparison of "Scan" tester parameters may be used to help locate intermittents and out-of-specification sensors.
If the "Scan" tester is not operating properly, check 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 "E" and the ECM. Also check for an open diagnostic test terminal from ALDL terminal "B" and ECM. With ignition on, the serial data line should have a between 2-5 volts and the diagnostic line about 5 volts.