Well where do I start. I have a 2008 F-250 6.4 FX-4. the other day my wife driver to work and everything is good, she gets in the truck to leave and nothing; no power to the starter. both bats are good, have power to everything else. Then a warning on the info screen that the trailer mod is in error. so I take both positive bat cables off and back on a few times and the truck starts great right not so much. we drive the truck home about five miles or so and it starts fine two more times, then no start and the same error on the trailer module. I did notice that the power does not seem to turn all the way off. when you kill the engine the stereo is on but no sound and stays that way and there's a whistle noise coming from the EGR valve assbly. the headlights will not shut off on auto, several icon appear in the display screen such as the oil can (engine not running key off) then low fuel icon, ect and they continue to flash. Strange I could go on and on what it's doing and all while the key is of. Oh yeah when the truck does start the stereo is on and no sound until you kill the truck and the sound comes on. It takes a while after it starts for everything to slowly come online such and power windows, info screen ect..
So what do you think? JUNK???

 

Check for DTCs (codes), they will give you a direction for further investigation. Keep in mind that most everything on these trucks is controlled via data messages sent on a CAN bus network, rather than old style individual wires. Disrupt that data network at any point in it's connections between modules and all kinds of symptoms will occur.

 

There is only one DTC that was not there before this began and it is glow plug heater circuit. I think this may be a result of the problem and not a contributing factor.