DTC P0148: Alternative Fuel Enable Control Circuit: Description
WARNING: This page is about a different car, the 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
The Powertrain Control Module (PCM) grounds the AF Enable circuit when operating on CNG. When the AF Enable circuit is grounded the AF relay is energized. The AF relay supplies power to all of the AF injector relays. The switched AF relay voltage also signals the Fuel Injector Control Module (FICM) to operate the CNG injectors based upon the PCM fuel injector control pulse width signals. The PCM opens the AF Enable circuit when gasoline operation is desired. Switch-over from one fuel to the other is always performed in an orderly, sequential manner. Since some injectors are in the middle of injecting the previous fuel, the FICM will wait until that cylinders fuel delivery is complete and will then switch over in sequential firing order.