Fuel Mode Operation
WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
The PCM grounds the AF Enable circuit when operating on CNG. When the AF Enable circuit is grounded the FICM will operate the CNG injectors based upon the PCM pulse width signals. The PCM also monitors the FICM status signal circuit for proper FICM operation.