Fuel Mode Operation
WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The PCM opens the AF enable circuit when operating on CNG. When the AF enable circuit is open the FICM will operate the CNG injectors based upon the PCM pulse width signals. The FICM will convert the FRT, FTP and FTT sensors voltage signals to PWM signals that the PCM monitors for fuel trim and fuel gage adjustment. The PCM also monitors the FICM status signal circuit for proper FICM operation.