I would like to add to this thread.
HAVE A GOOD LOOK AT THE WIRING HARNESS THAT RUNS ALONG THE LEFT FRAME RAIL TO THE REAR OF THE CHASSIS. LOOK ESPECIALLY CLOSE TO THE WIRES WHERE THE 2 HARNESS CONNECTIONS COME TOGETHER TO VERIFY THAT THERE AREN'T ANY BROKEN WIRES. Power to the fuel pump (located inside the fuel tank) feeds through one of those 2 connectors.
AGAIN... *** CHECK THE WIRING ALONG THE LEFT FRAME RAIL ***
This is what I found on my Ford truck:
My truck is a 2005 F150 XLT SuperCrew, 4.6 L V8. I encountered this code (DTC) P1233 - Fuel Pump Driver Module Offline recently. I took a look at the FPDM and decided to replace it due to the corrosion on it. This however was not my problem and I still could not get any power to my fuel pump. Then I started thinking the FUEL INERTIA SWITCH - I did a number of checks and could not 100% rule out whether it was the switch or not. I bypassed the switch at the wiring connector for the IFS. Still no power out to my fuel pump... scratching my head now! So then I figured I might as well try putting power (12VDC) direct to my fuel pump to see if it would run. In looking for my fuel pump wiring harness connector location on my truck, I come across the main rear chassis wiring harness running along the frame rail. This harness is between 3/4" to 1" diameter and is exposed on the outer side of the frame rail. About mid-cab (SuperCrew), there are 2 large harness connections - completely exposed to the elements. One of these locates the power connections to your tank internally-mounted fuel pump. It just so happens that I find 1 wire on one of these connectors that has completely broken away from the end of the connector - the wire insulator became very hard and brittle and cracked and the wire inside corroded over time and finally broke off. This trouble code can lead the average backyard mechanic to sink alot of $$$ and time replacing unnecessary parts... I've read in these forums where a number of owners had their trucks at dealerships for extended periods of time to correct similar problems and with much frustration and high repair bills (parts AND labor). Fortunately on my part, I lucked into finding a broken wire and my repair wasn't as extensive as some I've seen published throughout the internet. Good luck guys!