Possible Causes

WARNING: This page does not describe the selected car, but rather 9 other vehicles, including the 2003 Ford RV Cutaway, 2003 Ford Pickup, 2003 Ford F550 Super Duty, 2003 Ford F450 Super Duty, and 2003 Ford Excursion. However, it is still accessible from the selected car via links, so may be relevant.

A stuck open regulator valve will cause low fuel pressure.
A stuck closed regulator valve (possibly caused by debris) will cause high fuel pressure for a short time until pump fails.
A loose fuel line on suction side of fuel system can cause air to be ingested into system and cause low fuel pressure.
Electric fuel pump.
Fuel pump fuse.
Fuel pump relay.
Inertia Fuel Shutoff (IFS) switch.
Fuel pump circuit faults.