Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Diagnostic Circuit Check - Testing
The Diagnostic Circuit Check is an organized approach to identifying a problem created by an electronic engine control system alfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the technician to the next logical step in diagnosing the complaint.
The "Scan Data" may be used for comparison, after completing the DIAGNOSTIC CIRCUIT CHECK and finding the on-board diagnostics functioning properly and no trouble codes displayed. The typical values are an average of display values recorded from normally operating vehicles and are intended to represent what a normally functioning system would typically display.
Only the parameters listed are used in this article for diagnosis. If a "SCAN" tester reads other parameters the values are not recommended by General Motors for use in diagnosis.
If "SCAN" tester is not operating, check on another vehicle. If okay, the cigar lighter socket should be checked for 12 volts and a good ground. If "SCAN" tester reads "no data" or "no ALCL" with ignition on, check serial data wire for an open or short to ground between ALCL terminal "E" and ECM.
Also check for an open diagnostic test terminal for ALCL terminal "B" and ECM. With ignition on, serial data line (ALCL terminal "E") should have a varying 2-5 volts, and diagnostic line (ALCL terminal "B") about 5 volts.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.