Inertia Fuel Shutoff Switch (All Fuel Systems)
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Mountaineer, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, and 2003 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.
WARNING:
DO NOT reset IFS switch until complete fuel system has been inspected for leaks.
In the event of a collision or vehicle rollover, electrical contacts within the Inertia Fuel Shutoff (IFS) switch are tripped open when the internal steel ball breaks loose of the switch magnet. See Fig 1. Once loose, the steel ball rolls up a conical ramp and makes contact with the target plate which creates an open in the electrical circuit to electric fuel pump. If the electrical circuit is opened, it is not possible to restart the vehicle until the switch is reset. A reset button is located on top of IFS switch assembly.