Water-in-Fuel Indicator (diesel only)
WARNING: This page is about a different car, the 2005 Ford RV Cutaway, 2005 Ford Econoline, and 2005 Ford E450 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
The PCM receives a ground signal when water is sensed in the fuel. The PCM sends a message over the communication network to the instrument cluster, to illuminate the water-in-fuel indicator.