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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 143 (ENGINE CONTROLS - SYSTEM & COMPONENT TESTING)IGNITION SYSTEMELECTRONIC IGNITION SYSTEM DIAGNOSIS (4.2L)DIAGNOSTIC PROCEDURE
2003 Chevrolet TrailBlazer 4.2 S, 4WD

Diagnostic Procedure

2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Diagnostic Procedure
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2002 Saturn Vue, 2002 Pontiac Montana, 2002 Pontiac Aztek, 2002 Oldsmobile Silhouette, and 2002 Oldsmobile Bravada. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER XL article. After performing Diagnostic System Check - Engine Controls, go to next step.
  2. Are DTCs P0335 or P0336 set? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER XL article. If no, go to next step.
  3. Attempt to start engine. Does engine start and run? If yes, go to next step. If no, go to step  15.
  4. Observe Misfire Current Counters parameter with scan tool. Does scan tool display any Misfire Current Counters incrementing? If yes, go to next step. If no, go to DIAGNOSTIC AIDS .
  5. Remove ignition coil for affected cylinder. Install Spark Tester (J 26792) to spark plug boot. Attach clamp end of (J 26792) to a good engine ground. Observe (J 26792). Crank engine. Does spark jump tester gap? If yes, go to step  11. If no, go to next step.
  6. Turn OFF the ignition. Disconnect ignition coil for misfiring cylinder. Turn ON the ignition, with the engine OFF. Probe Ignition 1 voltage circuit of affected ignition coil with test light connected to good ground. Does test light illuminate? If yes, go to next step. If no, go to step  15.
  7. Probe Ignition 1 voltage circuit of affected ignition coil with test light connected to ignition coil ground circuit. Does test light illuminate? If yes, go to next step. If no, go to step  16.
  8. Start engine. Measure frequency from IC Control circuit of respective ignition coil to a good ground. Does frequency measure within 2-20 Hz? If yes, go to step  14. If no, go to next step.
  9. Turn ON the ignition, with engine OFF. Measure voltage from IC Control circuit of respective ignition coil to a good ground with DMM. Does voltage measure more than one volt? If yes, go to step  17. If no, go to next step.
  10. Test IC Control circuit for an open, a short to ground or high resistance. Did you find and correct the condition? If yes, go to step  21. If no, go to step  13.
  11. Exchange spark plug with a spark plug from a non-misfiring cylinder. Install ignition coil. Start engine. Does scan tool display that misfire followed that spark plug? If yes, go to step  20. If no, go to next step.
  12. Inspect insulating spark plug boot and conducting spring for contamination, deterioration or any arching. Did you find and correct the condition? If yes, go to step  21. If no, check for base engine mechanical faults. See appropriate TROUBLE SHOOTING article in ENGINES.
  13. Test for an intermittent and for a poor connection at PCM. Did you find and correct the condition? If yes, go to step  21. If no, go to step  19.
  14. Test for an intermittent and for a poor connection at ignition coil. Did you find and correct the condition? If yes, go to step  21. If no, go to step  18.
  15. Repair open or short to ground in Ignition 1 voltage circuit. After repairs, go to step  21
  16. Repair open in ignition coil ground circuit. After repairs, go to step  21
  17. Repair short to voltage in IC Control circuit. After repairs, go to step  21
  18. Replace ignition coil. After repairs, go to step  21
  19. Replace PCM. Program replacement PCM. See POWERTRAIN CONTROL MODULE under PROGRAMMING in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER XL article. After repairs, go to step  21
  20. Replace spark plug. After repairs, go to next step.
  21. Attempt to start engine. Does engine start and run? If yes, go to next step. If no, go to step  2.
  22. Clear DTCs with scan tool. Turn OFF the ignition for 30 seconds. Start engine. Let engine reach operating temperature. Are there any DTCs that have not been diagnosed? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER XL article.
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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.