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Home >> Pontiac >> 2001 >> Aztek Base, AWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - System & Component Testing >> Fuel System (CNG) >> Unwanted Switch To Gasoline >> Diagnostic Procedure
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Diagnostic Procedure

  1. Perform OBD system check. See ON BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article. Go to next step.
  2. Using scan tool, monitor AF ECU NGO enable input and output data parameters. Start engine. If NGO is enabled, go to next step. If NGO is not enabled, go to step  5.
  3. Check Fuel Indicator Light (FIL), located on fuel gauge select switch. If FIL is not illuminated, go to next step. If FIL is illuminated (gasoline operation), see FUEL GAUGE SELECTOR SWITCH/FUEL INDICATOR LIGHT .
  4. Fault is not present at this time. If problem still exists, perform OBD system check. See ON BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.
  5. Using scan tool, check fuel pressure sensor data. If pressure is 1200 psi or more, go to next step. If pressure is less than 1200 psi, go to step  10.
  6. Using scan tool, command NGO. Check NGO enabled data. If NGO is enabled, go to next step. If NGO is not enabled, go to step  13.
  7. Check fuel pressure sensor, engine coolant sensor and mixture control valve. If no problem is found, go to next step. If any problem is found, repair as necessary. After repairs, go to step  21.
  8. Check ignition sensors (CMP and CKP). Check secondary ignition system. Check all control module grounds. If no problem is found, go to next step. If any problem is found, repair as necessary. After repairs, go to step  21.
  9. Perform FUEL SYSTEM DIAGNOSIS (ALTERNATIVE FUEL) , then return to this step. If no repairs were required, go to step  20. If any repairs were required, go to step  21.
  10. Start engine. Using scan tool, command HPL open. If FPS data is less than 1200 psi, go to next step. If FPS data is 1200 psi or more, go to step  15.
    11. NOTE: Ensure manual lock-down screw on HPL manual shut-off fitting is open by turning Allen screw fully counter-clockwise.
  11. Ensure CNG tank is full by connecting vehicle to refilling station and filling tank. Turn ignition on. Compare filling station pressure reading to scan tool Fuel Pressure Sensor (FPS) data reading. If FPS is accurate, go to next step. If FPS is not accurate, see CNG FUEL GAUGE READING INACCURATE .
  12. Monitor FPS data. Start engine. Ensure vehicle is operating on CNG. Allow engine to idle for a few minutes. If FPS is ever less than 1200 psi, go to step  20. If FPS remains at 1200 psi or more, go to step  21.
  13. Using scan tool, monitor VCM NGO data. Turn ignition off. Disconnect AF ECU connectors. Using a fused jumper wire, connect AF ECU harness connector C001 terminal No. 3 (Pink wire) to ground. Turn ignition on. Check NGO enabled data. If NGO is not enabled, go to next step. If NGO is enabled, go to step  16.
  14. Check for open circuit on Pink wire between AF ECU harness connector C001 terminal No. 3 and VCM harness connector C4 terminal No. 2. Check for poor contact at terminals. Check VCM calibration. If no problem is found, go to step  17. If any problem is found, repair as necessary. After repairs, go to step  21.
  15. Check CNG system for leaks. Check low pressure lock off solenoid for pressure hold after shut-off. Check high pressure lock off solenoid for pulse prime at start-up. Repair as necessary. After repairs, go to step  21.
  16. Check contact at AF ECU and harness connector C001 terminal No. 3 (Pink wire). If no problem is found, go to step  18. If any problem is found, repair as necessary. After repairs, go to step  21.
  17. Replace VCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Program VCM while operating vehicle on gasoline. To ensure vehicle is operating on gasoline, remove CNG fuse (20-amp) from underhood fuse block. After repairs, go to step  21.
  18. Replace AF ECU. After repairs, go to step  21.
  19. Replace high pressure lock off solenoid. After repairs, go to step  21.
  20. Replace Gas Mass Sensor/Mixture Control Valve (GMS/MCV). After repairs, go to next step.
  21. Reconnect all components and connectors. Using scan tool, clear all DTCs. Turn ignition off for 30 seconds. Start engine. Test drive vehicle. If no problems are found, MIL operates properly and no DTCs are set, test is complete. If any problems are found, MIL does not operate properly or DTCs are set, perform OBD system check. See ON BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.
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