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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 114 (ENGINE CONTROLS SELF-DIAGNOSTICS - 2.2L CNG)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 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
  1. Were you sent here from other diagnostic service information? If yes, go to next step. If no, perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Turn ignition on, with engine off. Is the MIL continuously illuminated? If yes, go to next step. If no, go to step  5 .
  3. While communicating with the AF ECM, command the MIL OFF. Is MIL illuminated? If yes, go to MIL ON STEADY  . If no, go to next step.
  4. The fault is not present. Is an operational concern still present? If yes, perform DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. If no, problem is intermittent. Refer to INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  5. Crank the engine for at least 10 seconds. Does the engine start and run? If yes, go to next step. If no, go to step  9 .
  6. Observe the operation of the other Instrument Panel Cluster (IPC) gauges and/or lights. Do all the IPC indicators, except the MIL, operate correctly? If yes, go to next step. If no, go to step  13 .
  7. Turn ignition on, with engine off. Observe the Gasoline MIL Command data parameter in the Alt. Fuels Data List. Is the PCM commanding the MIL on? If yes, go to next step. If no, go to step  11 .
  8. Turn ignition OFF and wait 30 seconds. Disconnect the AF ECM connectors. Turn ignition on, with engine off. Connect a fused jumper between the MIL control (AF ECM) circuit terminal and a known-good ground. If the jumper fuse opens, repair short to voltage on the AF ECM MIL control circuit. Is the MIL illuminated? If yes, go to step  14 . If no, go to step  10 .
  9. The fault is in PCM system. Repair as necessary. After repairs, go to step  17 .
  10. Test the MIL control circuit between the AF ECM connector terminal and the IPC for an open. Repair the circuit as necessary. Was a circuit repair necessary? If yes, go to step  17 . If no, go to step  13 .
  11. Test the MIL INPUT circuit between the AF ECM connector terminal and the PCM connector terminal for an open or a short to voltage. Test the MIL control (AF ECM) circuit between the AF ECM connector terminal and the instrument panel for a short to voltage. Repair the circuit as necessary. Was a circuit repair necessary? If yes, go to step  17 . If no, go to next step.
  12. Check terminal contact at the PCM connector. Repair terminal contact as necessary. Was a terminal repair necessary? If yes, go to step  17 . If no, go to step  15 .
  13. The fault is in the instrument panel. Diagnose and repair problem in IPC. After repairs, go to step  17 .
  14. Check terminal contact at the AF ECM connector. Repair terminal contact as necessary. Was a terminal repair necessary? If yes, go to step  17 . If no, go to step  16 .
    15. NOTE: The following control module replacement procedures must all be completed before attempting to operate vehicle.
  15. Replace PCM. See POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CARS article. Program (flash) PCM with correct vehicle software and calibration. See POWERTRAIN CONTROL MODULE REPROGRAMMING  under PROGRAMMING. To perform CKP variation learn procedure, a switchover to gasoline operation is necessary. Remove CNG fuse (20-amp) located in underhood fuse block to perform switchover. Perform CKP system variation learn procedure. See CRANKSHAFT POSITION SENSOR SYSTEM VARIATION LEARN PROCEDURE  under PROGRAMMING. After performing repairs and procedures, go to step  17 .
  16. Replace AF ECM. See ALTERNATE FUEL ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CARS article. Program (flash) AF ECM with correct vehicle software and calibration. See ALTERNATE FUEL ENGINE CONTROL MODULE  under PROGRAMMING. After performing repairs and procedures, go to next step.
  17. Remove all test equipment except the scan tool. Connect any disconnected components, fuses, etc. Using scan tool, clear DTC information from AF ECM and Powertrain Control Module (PCM). Turn ignition OFF and wait 30 seconds. Start engine and operate vehicle. Observe the fuel indicator light and MIL. Observe vehicle performance and driveability. After operating vehicle, inspect BOTH control modules for stored DTCs. Does vehicle operate on CNG with normal driveability, no FIL/MIL illumination and no stored DTCs? If yes, system is okay. If no, perform DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
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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.