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.
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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 89 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.1L ENGINE)SYSTEM TESTSMIL INOPERATIVEDIAGNOSTIC PROCEDURES
2003 Chevrolet TrailBlazer 4.2 S, 4WD
Diagnostic Procedures
2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Pontiac Grand Prix, 2002 Chevrolet Malibu, and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
- Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
- Attempt to start engine. Does engine start? If yes, go to next step. If no, go to step 9.
- Inspect the fuse supplying voltage to the MIL. Is fuse open? If yes, go to step 7. If no, go to next step.
- Turn ignition off. Disconnect the PCM connector containing the MIL control circuit. Turn ignition on, with engine off. Probe MIL control circuit in the PCM harness connector with a fused jumper wire connected to ground. Is the MIL ON? If yes, go to step 15. If no, go to next step.
- Turn ignition off. Remove IPC. Probe the MIL ignition positive voltage circuit at IPC harness connector with a test light connected to a good ground. See WIRING DIAGRAMS article. Is test light illuminated? If yes, go to next step. If no, go to step 8.
- Test the MIL control circuit for an open or high resistance and repair as necessary. Did you find and correct the condition? If yes, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS . If no, go to step 14.
- Repair the short to ground in the MIL ignition positive voltage circuit. After repairs, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS .
- Repair the open in the MIL ignition positive voltage circuit. After repairs, DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS .
- Inspect the PCM fuse and the ignition 1 main fuse. Is one or both of the fuses open? If yes, go to step 12. If no, go to next step.
- Turn ignition off. Disconnect the PCM. Turn ignition on, with engine off. Connect a test light between the PCM ignition positive voltage circuits and a good ground. Does the test light illuminate while probing ALL PCM ignition positive voltage circuits? If yes, go to next step. If no, go to step 12.
- Connect test light from PCM battery positive voltage circuit and good ground. Does the test light illuminate? If yes, go to step 13. If no, go to next step.
- Locate and repair the open or short to ground in the PCM ignition feed circuits or the PCM battery positive voltage circuit. After repairs, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS .
- Inspect for a poor PCM ground or a poor PCM ground connection. Did you find and correct the condition? If yes, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS . If no, go to step 15.
- Inspect for poor connections at the IPC and repair as necessary. Did you find and correct the condition? If yes, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS . If no, go to step 16.
- Inspect for poor connections at the PCM and repair as necessary. Did you find and correct the condition? If yes, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS . If no, go to step 17.
- Replace the IPC. After repairs, DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS .
- Replace PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE under PROGRAMMING. After repairs, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS .
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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.