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Diagnostic Procedure

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  1. Perform Alternative Fuels Diagnostic System Check - Engine Controls. See ALTERNATIVE FUELS DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article. After performing Alternative Fuels Diagnostic System Check - Engine Controls, go to next step.
    2. NOTE: Verify customers concern is not a normal operating condition. See DIAGNOSTIC AIDS .
  2. Is customers concern a normal operating condition? If yes, go to INTERMITTENTS in TROUBLE SHOOTING - NO CODES - TRUCKS article. If no, go to next step.
  3. Turn ON the ignition, with the engine OFF. Observe Alternative Fuel Level data display from AF ECM with scan tool. Observe Instrument Panel (IP) fuel gauge. Does IP fuel gauge accurately display Alternative Fuel Level? If yes, go to step  10. If no, go to next step.
  4. With scan tool, communicate with AF ECM and observe AFO Signal and AFO Signal Command data parameters. Observe data parameters as you crank and start engine. Do both data parameters indicate AF ECM is in control of engine fueling? If yes, go to next step. If no, go to FUEL SYSTEM UNWANTED SWITCH TO GASOLINE (BI-FUEL ONLY) .
  5. With scan tool, communicate with IP. Command fuel gauge between Full and Empty. Does fuel gauge operate as commanded? If yes, go to next step. If no, perform DIAGNOSTIC SYSTEM CHECK - INSTRUMENT CLUSTER in appropriate ANALOG INSTRUMENT PANEL article.
  6. Is this vehicle equipped with Bi-fuel (KL6) option? If yes, go to next step. If no, go to step  15.
  7. Turn ON the ignition, with the engine OFF. Observe Fuel Gauge Selected data display from PCM with scan tool. Press fuel gauge select switch. Does Fuel Gauge Selected data display change when switch is Pressed? If yes, go to step  15. If no, go to next step.
  8. Turn OFF the ignition. Disconnect PCM connectors. Turn ON the ignition, with the engine OFF. Probe Fuel Gauge Select Switch Signal circuit with a test light connected to battery voltage. Is test light illuminated? If yes, go to step  17. If no, go to next step.
  9. Press fuel gauge select switch while observing test light. Does test light illuminate when switch is pressed? If yes, go to step  23. If no, go to step  18.
  10. Observe Fuel Temperature Sensor voltage display. Does display indicate a voltage more than 4.8 volts? If yes, go to step  20. If no, go to next step.
  11. Observe Fuel Tank Pressure Sensor voltage display. Does display indicate a voltage more than 4.8 volts? If yes, go to step  21. If no, go to next step.
    12. NOTE: Fast-filling CNG fuel tank causes fuel to heat-up and expand. Fuel temperature may remain hotter than ambient Intake Air Temperature (IAT) for up to 3 hours after filling.
  12. Observe Fuel Temperature Sensor and IAT Sensor data displays with scan tool. Compare temperature displays. Does each sensor indicate the same temperature plus or minus 35°F (20°C)? If yes, go to next step. If no, go to step  25.
    13. NOTE: Fast-filling CNG fuel tank causes fuel to heat-up and expand. Fuel tank pressure may decrease by up to 800 psi within 3 hours of fast filling.
  13. Observe Fuel Tank Pressure Sensor data display with scan tool. Verify amount of CNG fuel pressure in fuel tank by filling CNG fuel tank at CNG dispensing station. Observe Fuel Tank Pressure Sensor data display after tank is filled. Does FTP sensor indicate fuel pressure near 3600 psi (24,820 kPa) at 70°F (21°C)? If yes, go to next step. If no, go to step  26.
  14. Start and idle engine. Observe Fuel Tank Pressure Sensor data display with scan tool. Command High Pressure Lock-Off (HPL) solenoid OFF while monitoring Fuel Tank Pressure Sensor data display. Observe Fuel Tank Pressure Sensor data display until engine stalls or switches to gasoline operation. Does Fuel Tank Pressure Sensor data display smoothly indicate fuel pressure decrease and also indicate fuel pressure less than 200 psi (1379 kPa)? If yes, go to INTERMITTENTS in TROUBLE SHOOTING - NO CODES - TRUCKS article. If no, go to step  26.
  15. Turn OFF the ignition. Disconnect AF ECM. Turn ON the ignition, with the engine OFF. Measure voltage between Alternative Fuel Level Control circuit and ground with DMM (J 39200). Is voltage 4.8-5.2 volts? If yes, go to next step. If no, go to step  19.
  16. Connect Alternative Fuel Level Control circuit to ground with a fused jumper. On Bi-fuel (KL6) vehicles press fuel gauge select switch. Observe fuel gauge immediately after jumpering circuit or pressing switch. Does fuel gauge display near empty? If yes, go to step  24. If no, go to step  27.
  17. Disconnect cargo/fuel gauge select switch. Probe Fuel Gauge Select Switch Signal circuit with test light connected to battery voltage. Is test light illuminated? If yes, go to step  22. If no, go to step  29.
  18. Test Fuel Gauge Select Switch Signal circuit between PCM connector terminal and switch for an open or short to voltage. Did you find and correct the condition? If yes, go to step  30. If no, go to step  29.
  19. Test Alternative Fuel Level Signal circuit between PCM connector terminal and AF ECM for an open, short to ground or short to voltage. Did you find and correct the condition? If yes, go to step  30. If no, go to step  23.
  20. Test Fuel Tank Temperature Sensor Signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step  30. If no, go to step  25.
  21. Test Fuel Tank Pressure Sensor Signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step  30. If no, go to step  26.
  22. Repair Fuel Gauge Select Switch Signal circuit for a short to ground condition. After repairs, go to step  30.
  23. Inspect for proper terminal contact at PCM connector. Did you find and correct the condition? If yes, go to step  30. If no, go to step  27.
  24. Inspect for proper terminal contact at AF ECM connector. Did you find and correct the condition? If yes, go to step  30. If no, go to step  28.
  25. Replace Fuel Tank Temperature (FTT) sensor. FTT sensor is not serviceable separately from HPL. After repairs, go to step  30.
  26. Replace FTP sensor. After repairs, go to step  30.
    27. NOTE: When replacing PCM, Bi-fuel (LQ4/KL6) models require a switchover to gasoline with scan tool in order to perform Crankshaft Position Variation Learn Procedure. CNG only (LQ4/KL8) models DO NOT require Crankshaft Position Variation Learn Procedure.
  27. Replace PCM. Perform PCM relearn procedures. See PROGRAMMING in appropriate SELF-DIAGNOSTICS article. After repairs, go to step  30.
  28. Replace AF ECM. After repairs, go to step  30.
  29. Replace fuel gauge select switch. After repairs, go to step go to next step.
  30. Connect any disconnected components, fuses, etc. With scan tool, clear DTC Information from AF ECM and PCM. Turn the ignition OFF and wait 30 seconds. Start engine and operate vehicle. Observe MIL and Fuel Indicator Lamp (KL6 only). Observe vehicle performance and driveability. After operating vehicle, inspect AF ECM and PCM 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 Alternative Fuels Diagnostic System Check - Engine Controls. See ALTERNATIVE FUELS DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.
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