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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. 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.
  2. Attempt to start engine. Will the engine start and run? If yes, go to next step. If no, diagnose fuel lock-off relay. See FUEL LOCK-OFF RELAY DIAGNOSIS (CAVALIER - CNG) under FUEL SYSTEM in SYSTEM & COMPONENT TESTING - CARS - EXCEPT METRO & PRIZM article.
  3. Turn ignition on, with engine off. Observe the Service Engine Soon Malfunction Indicator Light (MIL). Is the MIL illuminated? If yes, go to next step. If no, go to MIL INOPERATIVE  under SYSTEM TESTS.
  4. Using scan tool, select SPECIAL FUNCTION display for alternative fuel and command AF ECM to turn MIL off. Is the MIL illuminated when commanded OFF? If yes, go to MIL ON STEADY  under SYSTEM TESTS. If no, go to next step.
  5. Turn ignition on, with engine off. Using the Engine Data, Alt. Fuel, data display, observe the AFO signal and AFO Signal Command displays. Do both displays indicate the same information? If yes, go to next step. If no, go to step  9 .
  6. Turn ignition on, with engine off. Capture the DTC Information (stored when this DTC sets) from the AF ECM into the scan tool. Clear the DTC Information from the AF ECM. Using the Special Function, Alt. Fuel, display command the AFO Enable back and forth between gasoline and Alt. Fuel. Observe the AFO Signal and the AFO Signal Command displays while commanding. Do both displays indicate the same information? If yes, go to next step. If no, go to step  9 .
  7. Exit the Special Function area of the scan tool. Start and idle the engine. Observe the AFO Signal and the AFO Signal Command displays. Is the engine operating on Alternative Fuel? If yes, go to next step. If no, go to step  12 .
  8. The fault is not present. See DIAGNOSTIC AIDS  . Are any DTCs stored that have not been diagnosed? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no, refer to INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM under SELF-DIAGNOSTIC SYSTEM.
  9. Observe the AFO Signal. Does the AFO Signal display indicate gasoline? If yes, go to next step. If no, go to step  11 .
  10. Turn ignition off. Disconnect the AF ECM connectors. Using a fused jumper connect the AF ECM connector AFO Enable circuit terminal to a known-good ground. If the jumper fuse blows the AFO Enable circuit is shorted to voltage. Turn ignition on, with engine off. Using scan tool, observe the AFO Signal data display located in the Engine Data Gasoline list. Does the AFO Signal display indicate Alternative Fuel is enabled? If yes, go to step  21 . If no, go to step  16 .
  11. Turn ignition off. Disconnect the AF ECM connectors. Turn ignition on, with engine off. Using scan tool, observe the AFO Signal data display located in the Engine Data Gasoline list. Does the AFO Signal display indicate Gasoline is enabled? If yes, go to step  22 . If no, go to step  17 .
  12. Backprobe the Lock-Off Relay switch load terminal (87) with the J-34142-B 12-Volt using a test light connected to a known-good ground. Start and idle the engine. Is the test light illuminated while the engine is running? If yes, go to next step. If no, see FUEL LOCK-OFF RELAY DIAGNOSIS (CAVALIER - CNG) under FUEL SYSTEM in SYSTEM & COMPONENT TESTING - CARS - EXCEPT METRO & PRIZM article.
  13. Turn ignition off. Disconnect AF ECM connectors. Disconnect the lock-off relay connector. Relay is located on left front corner of engine compartment, behind headlight. Connect the positive test lead of Digital Multimeter (DMM) to the lock-off relay switch load terminal (No. 87) at harness connector. Connect DMM negative test lead to the low pressure lock-off solenoid control circuit terminal of AF ECM connector (C002). Measure resistance of LPL circuit. Is resistance within 6.2-10.0 ohms? If yes, go to next step. If no, go to step  18 .
  14. Retain the positive test lead of DMM connected to lock-off relay switch load terminal (87) at the harness connector. Connect negative test lead to the high pressure lock-off solenoid control circuit terminal (3) of AF ECM connector (C002). Measure resistance of HPL circuit. Is resistance within 16.3-20.6 ohms? If yes, go to next step. If no, go to step  18 .
  15. Measure and record the resistance between the LPL solenoid control circuit terminal (1) and a known-good ground. Measure and record the resistance between the HPL solenoid control circuit terminal (3) and a known-good ground. Does either circuit measure less than infinite? If yes, go to step  20 . If no, go to step  22 .
  16. Inspect the AFO enable circuit for an open or high resistance condition. Inspect the PCM connector for proper terminal contact. Repair wiring or terminals as necessary. Was a wiring or terminal repair necessary? If yes, go to step  24 . If no, go to step  23 .
  17. Inspect the AFO enable circuit for a shorted to ground condition. Repair wiring as necessary. Was a wiring repair necessary? If yes, go to step  24 . If no, go to step  23 .
  18. If resistance measured greater than the specified value range, check for disconnected solenoid connector and test for control circuit continuity between the solenoid connector terminal "B" and AF ECM connector terminal. Check for power feed circuit continuity between the lock-off relay connector terminal and solenoid connector terminal. If the resistance measured is less than the specified value range, check for a disconnected solenoid connector and test for a shorted AF diode. Check for short between solenoid control circuit and solenoid power feed circuit. Repair wiring as necessary. Was a wiring repair or AF diode replacement necessary? If yes, go to step  24 . If no, go to next step.
  19. Replace suspect lock-off solenoid. See LOCK-OFF SOLENOIDS under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CARS article. After repairs, go to step  24 .
  20. Repair the shorted to ground solenoid control circuit. After repairs, go to step  24 .
  21. Inspect the AF ECM connectors for proper terminal contact. Repair terminal contact as necessary. After repairs, go to step  24 .
  22. 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 step  24 .
  23. Replace PCM. See POWERTRAIN CONTROL MODULE in appropriate REMOVAL, OVERHAUL & INSTALLATION article. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE REPROGRAMMING  under PROGRAMMING. After replacing PCM, go to next step.
  24. Using scan tool, clear DTCs from AF ECM and PCM. Start engine and allow to idle until engine reaches normal operating temperature. Select DTC and specific DTC function. Enter DTC number and operate vehicle within conditions for setting this DTC, until scan tool indicates diagnostic has run. See CONDITIONS FOR SETTING DTC  . If DTC diagnostic runs and passes, system is okay. If DTC resets, go to step  2 .
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