FICM Diagnostic Circuit
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 monitors the FICM diagnostic circuit in order to set FICM related DTCs. The FICM diagnostic circuit is a 5-volt circuit that is pulse width modulated by the FICM in order to communicate diagnostic information to the PCM.