Diagnostic Aids

Diagnostic information should be utilized only if concern of vehicle operator can be duplicated. An unwanted switch may occur only during specific vehicle operation. Examples of unwanted switch are as follow:

• Vehicle has an intermittently leaking Low Pressure Lock-Off (LPL) solenoid.
• After turning engine OFF and waiting several minutes, all CNG purges from fuel lines.
• During next ignition ON key cycle, High Pressure Lock-Off (HPL) solenoid is opened for approximately one second. At the same moment HPL is opened, AF ECM monitors fuel pressure sensor. If LPL is still leaking, CNG pressure cannot build within the system and low fuel pressure is observed by AF ECM. Detecting low fuel pressure in the system, AF ECM performs a switch-over to gasoline.
• Vehicle may start and run on CNG during any ignition cycle in which LPL does not leak. Customer concern would be an intermittent unwanted switch to gasoline with no DTCs stored and possibly a no-start flooded condition.

If an unwanted switch only occurs during certain driving conditions, perform fuel system diagnosis and verify that fuel pressure can be maintained during all driving conditions. See FUEL SYSTEM DIAGNOSIS .