I dont post stuff often but I like to post something when I have a fix to share. I had a SERVICE TRAILER BRAKE SYSTEM message coming on after start up without a trailer hooked up. It came on evertime I started my truck. The funny thing was that if I hooked up my trailer I would NOT get the message and everything worked perfect. But when I disco'd the trailer again the error message would come back. The fix---There was a programming error (dont know how or why it happened) but I had it reprogrammed and it's been right as rain ever since! Just thought I'd share something quick to try before you go for a controller or relay or sensor. I have been finding that the dealers are going for the "bolt-ons" like these first thing......of course!! Hope this helps somebody out there!