This 2003 Ford F150 came in with the complaint that the engine would not fire up and run. The starter worked, the theft light flashes quickly and the odometer reads all dashes instead of numbers. Scanning the PCM found that it would not communicate with the scan tool. There was a code U1262 ( missing standard corporate protocol communication bus malfunction) stored in the IPC/HEC (Instrument Panel Cluster /Hybrid Electronic Cluster). Basically the IPC/HEC set a code because it knows that another computer module is not talking to it. An interesting note about the flashing theft light. Turn the ignition switch to the run position and watch the theft light as it flashes quickly for one minute. After the 60 second period of the theft light flashing rapidly the light will go out. Then it may start flashing slowly in a pattern. One flash, a pause and then 6 quick flashes. the pattern will repeat. The pattern is a two digit code of one followed by a six. The code is a 16 and is equivalent to a U1147 or a U1262. I already had pulled the code U1262 with my scan tool.
All of the evidence is pointing to a problem with the PCM not communicating. The next step is to check the voltage on the red wire at the idle air control (IAC) solenoid assembly, the mass air flow (MAF) sensor or at any injector connector. with the ignition on. There should be a nominal 12 volts present. The reading I got was .378 volts.
The PCM power relay (pointing to below) supplies power to the red wire mentioned above. Installing a new relay restored battery voltage to the red wire circuits.
Testing the output signal of the PCM power relay can also be done at several fuses in the underhood fuse/relay box. Fuse number 18.
Fuse number 23.
Switched ignition power for terminal 86 of the relay comes from the #30 fuse (30 amp) in the central junction box / interior fuse box.
I also replaced the fuel pump relay at the same time since it will likely fail in the near future.
The engine now started and ran, the theft light went out and the digital odometer reading was restored. With the mileage on this vehicle I should also point out that the odometer is sometimes just blank. That is a completely different problem and I have covered it here.