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 test. Failure to perform the System Check will result in misdiagnosis.
2. 2
   
   This step checks the battery voltage supply from the lock-off relay to the LPL. The lock-off relay is energized regardless of which fuel system is operating the vehicle.
3. 3
   
   This step checks the AF ECM and the Low Pressure Lock-Off Solenoid Control circuit for proper operation. The test lamp should illuminate for a few seconds then turn OFF as the engine cannot start on CNG with the LPL disconnected.
4. 4
   
   This step checks the battery voltage supply from the lock-off relay to the HPL. The lock-off relay is energized regardless of which fuel system is operating the vehicle.
5. 5
   
   This step check the AF ECM and the High Pressure Lock-Off Solenoid Control circuit for proper operation. The test lamp should illuminate for a few seconds then turn OFF as the AF ECM opens the HPL for system priming prior to cranking the engine.
6. 6
   
   This step checks for proper battery voltage output from the lock-off relay.
7. 7
   
   This step checks for proper relay coil positive voltage and ground supply.
8. 8
   
   This step checks for proper battery voltage supply from the CNG fuse (20-amp) circuit to the lock-off relay. If the test lamp fails to illuminate the failure can only exist between the relay and the circuit splice. For example: a blown fuse would cause a lack of power to the AF ECM causing no scan tool communication in which the OBD System Check would have directed you to a different diagnostic procedure.
9. 9
   
   This step checks for proper positive voltage supply from the gasoline fuel pump circuit to the lock-off relay.