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Diagnostic Procedure

  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. Start and idle engine. Is Fuel Indicator Light (FIL) illuminated? If yes, go to next step. If no, go to INTERMITTENTS in TROUBLE SHOOTING - NO CODES - TRUCKS article.
  3. Observe operation of other Instrument Panel (IP) gauges and/or lights. Do all IP indicators, except FIL, operate correctly? If yes, go to next step. If no, perform Diagnostic System Check - Instrument Cluster. See appropriate ANALOG INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT.
  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. Turn OFF the ignition. Disconnect PCM connectors. Turn ON the ignition, with the engine OFF. Is fuel indicator light illuminated? If yes, go to next step. If no, go to step  7.
  6. Test FIL Control circuit between PCM connector terminal and IP for a short to ground. Did you find and correct the condition? If yes, go to step  8. If no, perform Diagnostic System Check - Instrument Cluster. See appropriate ANALOG INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT.
    7. 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.
  7. Replace PCM. Perform PCM relearn procedures. See PROGRAMMING in appropriate SELF-DIAGNOSTICS article. After repairs, go to next step.
  8. Remove all test equipment except scan tool. 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. Observe FIL. Start engine and operate vehicle. Does FIL operate for a 2 second bulb check and then turn OFF? 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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