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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 370 (Engine Control System - Self-Diagnostics - 4.2L) >> Diagnostic Tests >> DTC P0301-P0306: Cylinder Misfire Detected >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: You must perform the Crankshaft Position (CKP) system variation learn procedure before proceeding with this diagnostic test. Refer to CRANKSHAFT POSITION SENSOR  under PROGRAMMING.
  2. Start engine. Allow engine to idle or operate within the conditions listed in the Freeze Frame/Failure Records. Monitor all of the Misfire counters with the scan tool. Are any of the Misfire current counters incrementing? If yes, go to next step. If no, problem is intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
  3. Are any other DTCs set? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  4. Can any abnormal engine noise be heard? If yes, diagnose engine mechanical condition. If no, go to next step.
    5. NOTE: Not grounding the ignition coil may cause an erratic spark.
  5. Turn ignition off. Remove the fuel pump relay. Remove the ignition coil for the affected cylinder, keeping the ignition connected to the harness connector. Connect a jumper wire between the ignition coil and a good ground. Install Spark Tester (J-26792) on the ignition coil and a good ground. Crank engine. Does the spark tester spark? If yes, go to step  7. If no, go to next step.
  6. Inspect the affected cylinder spark plug boot for a missing or damaged ignition coil spring. Did you find and correct the condition? If yes, go to step  15. If no, diagnose Electronic Ignition (EI) System. See ELECTRONIC IGNITION SYSTEM DIAGNOSIS under IGNITION SYSTEMS in SYSTEM & COMPONENT TESTING - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
  7. Remove the spark plug from the cylinder that indicated a misfire. Inspect the spark plug. Does the spark plug appear to be okay? If yes, go to next step. If no, go to step  9.
  8. Exchange the suspected spark plug with another cylinder that is operating properly. Operate the vehicle under the same conditions that the misfire occurred. Did the misfire move with the spark plug? If yes, go to step  14. If no, go to step  12.
  9. Is the spark plug oil or coolant fouled? If yes, diagnose for an engine mechanical problem. If no, go to next step.
  10. Is the spark plug gas fouled? If yes, go to step  12. If no, go to next step.
  11. Did the spark plug show any signs of being cracked, worn, or improperly gapped? If yes, go to step  13. If no, go to next step.
  12. Perform the fuel injector coil test. Did you find and correct the condition? If yes, go to step  15. If no, diagnose for an engine mechanical condition.
  13. Replace or gap the spark plug. After repairs, go to step  15.
  14. Replace faulty spark plug. After replacing spark plug, go to next step.
  15. Was the customer's concerned about the MIL flashing? If yes, go to next step. If no, go to step  17.
  16. Operate the vehicle at 2500 RPM for 4 minutes. Operate the vehicle within the Conditions For Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to DTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1) .
  17. Clear the DTCs with scan tool. Turn ignition off for 30 seconds. Start engine. Operate vehicle within the Conditions For Running DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  18. Observe the stored information, Capture Info with scan tool. Does scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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