My 2008 Ford F-150 5.4L XLT 4X4 started and ran fine today until I returned home from work. I had to run an errand and it did not start or turn over. My initial thought was a low battery voltage but everything else was working fine and the fact I changed the alternator a few months back told me otherwise. After doing a search on the computer, I swapped out the PCM relay #203 with another of the same size. It then started up. I shut it off immediately thinking I solved the issue. Upon starting it, nothing happened just like before.

 

I would have to conclude that the relay wasn't the problem after all.