So, a buddy of mine has a 2000 F250 Superduty with the 5.4l V8. He gave me a ring because it wasn't starting, all it would do when you turned the key is click once. I went and took a look at it, the click seemed to be coming from the starter solenoid. We replaced the solenoid, and the same thing happens, it just clicks once from the solenoid and that is it.

Any idea what might be going on?

 

the starter relay. it happened to my jeep I replaced it and my problem was solved