Test Description

The number below refer to the step number on the diagnostic procedure.

1. 6
   
   This step determines if the Gas Mass Sensor/Mixture Control Valve (GMS/MCV) connector and motor connector terminals and wiring are okay.
2. 7
   
   This step tests for the proper ignition voltage to the GMS/MCV.
3. 8
   
   This step tests for the proper ground circuits to the GMS/MCV.
4. 9
   
   This step tests the actual gas circuit for continuity to the AF ECM. The AF ECM supplies about 5 volts on the actual gas circuit.
5. 10
   
   This step tests the desired gas circuit for continuity to the AF ECM. With ignition on, with engine off, the AF ECM grounds the desired gas circuit.
6. 11
   
   This step tests the desired gas circuit for a short to ground or a defective AF ECM. With the ignition OFF, the AF ECM can no longer ground the desired gas circuit and the test light should turn OFF.
7. 12
   
   This step tests for proper initial system pressures. Any unexpected loss of fuel pressure may cause the GMS/MCV to output a low frequency signal and set this DTC. The AF ECM will open the lock-off solenoids and attempt to start on CNG every engine crank even when this DTC is stored. CNG fuel pressure might only be observed momentarily if this DTC sets during or shortly after engine cranking. An ignition cycle is required to reset the AF ECM.
8. 13
   
   This step tests the AF ECM's ability to generate a sufficient desired gas frequency signal.
9. 14
   
   This step tests the GMS/MCV's ability to generate a sufficient actual gas frequency signal.
10. 20
    
    This step tests for a defective GMS/MCV or AF ECM. The GMS/MCV supplies about 5 volts on the desired gas circuit.