Test Description

The numbers below refer to the step numbers in the diagnostic procedures.

1. 1
   
   The System Check must be performed before proceeding with this diagnostic procedure. Failure to perform the System Check will result in misdiagnosis.
2. 2
   
   This step inspects if the AF ECM has commanded the gasoline control module, using the AFO Enable circuit, to start on CNG.

   The AFO Signal Command parameter is the commanded state of the AFO Enable circuit from the AF ECM. The AFO Signal parameter is the monitored state of the AFO Enable circuit from the GASOLINE control module (PCM).

3. 3
   
   This step inspects if the FIL is illuminated during CNG operation. The FIL should illuminate only during gasoline operation.
4. 5
   
   This step inspects if the FTP sensor is monitoring adequate fuel tank pressure for the vehicle to start on CNG.
5. 6
   
   This step tests the ability of the AF ECM to command Alternative Fuels Operation (AFO) and the gasoline control modules ability to monitor the command.
6. 7
   
   This step indicates all the possibilities that cause a switch-over to gasoline operation.
7. 8
   
   This step inspects the ignition system of the vehicle for proper operation. A vehicle that operates correctly on gasoline but not on CNG may have a faulty ignition system.
8. 9
   
   Performing the Fuel System Diagnosis checks the basic AF fuel system for proper pressures and lock-off solenoid operation.
9. 10
   
   A fuel system that is unable to retain fuel pressure after the ignition is turned OFF may cause an Unwanted Switch to Gasoline. The AF ECM commands the HPL open for about 1 second every time the ignition is turned ON. If the fuel line system leaks during ignition OFF the next ignition ON prime pulse will usually recharge the fuel lines and the FTP sensor. A sluggish HPL may not open enough during the 1 second prime pulse but will open when commanded with the scan tool.
10. 11
    
    This step tests for proper fuel tank pressure sensor operation.
11. 12
    
    This step tests for an inoperative HPL. After connecting to a CNG dispensing station the CNG fuel lines, the FTP sensor and the fuel tank will be charged by the station. The CNG fuel pressure in the lines and the FTP sensor will quickly drop if the engine is operating on CNG but the HPL fails to open.
12. 13
    
    This step tests for proper operation of the AFO Enable circuit and the gasoline control module.
13. 15
    
    This step tests for a leaking fuel system with a sluggish HPL.