I have been having a problem the last few months with my 2008 F-150 5.4L...
Usually within 24hrs or less of sitting the truck refusses to start regardless of the remote start or just the key. I have taken it to the FORD garage and they dont have any idea what the problems is. Battery and alternator have check out...with no apparent problems. They said it might be the accesories I added on. They should not be draining the battery, I have a relay in place that appears to be working. While in the shop the truck started like a dream. It appears to happen more frequently when its cold outside. Tonight was the worse it has been, used the remote start to start the vehicle. When I went outside the truck was dead and everything was on. The vehicle then proceeded to not power down when the door was open. I then hooked the truck up to my booster, and everything shut down. The battery was so dead, I was unable to start it within 15 mins. So I left the truck there, and got a ride to work. Speaking with other people, they had similar problems which was caused by the remote start drawing the battery down. I really dont think it is my own wiring...there is no power after the relay when the truck is off.

Any Ideas????

 

You are going to have to disconnect all your addons to troubleshoot this properly. Even if you think your isolation relay is working properly, it's stuff that didn't come with the truck and you can't expect a dealer to be able to figure out something that's non-stock.

 

Thats exactly what I did. Seems o be a problem with or without my add-ons. It is frustrating because it is so random...

 

With all the addons disconnected, NOW take it back to the dealer.