Test Description

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

1. 1
   
   The Diagnostic System Check - Engine Controls must be performed before proceeding with this diagnostic test. Failure to perform the OBD System Check will result in misdiagnosis.
2. 2
   
   This step tests if the engine will start and run. A failure that causes the engine to not start on either fuel can be isolated to the CNG harness or a base vehicle problem such as a malfunctioning gasoline fuel pump relay.
3. 3
   
   This step tests the gasoline control modules ability to supply power to the gasoline fuel pump circuit.
4. 5
   
   This step tests for voltage through the AF fuel pump relay. With the ignition ON and the engine OFF the AF ECM does not energize the AF fuel pump relay allowing voltage to flow through the relay and energize the gasoline fuel pump.
5. 6
   
   This step tests the AF ECMs ability to energize the AF fuel pump relay and disable the gasoline fuel pump during CNG operation.
6. 7
   
   This step tests for stuck AF fuel pump relay contacts or a shorted AF Fuel Pump Relay control circuit.
7. 10
   
   If the engine did not operate on gasoline during the Diagnostic System Check - Engine Controls but now operates on gasoline inspect for intermittent terminal contact or a AF fuel pump relay with sticking contacts that may have been disturbed during testing.
8. 11
   
   This step tests if the CNG harness is the cause of the no-start on gasoline.
9. 15
   
   If the Fuel Pump Power circuit tests okay there is a base vehicle problem that is causing the no-start condition.