What exactly did the message say?
If it said something like "Check trailer brake system" that means a problem on the truck itself, not the trailer. I think the message is different if it senses a problem with the trailer itself - maybe "check trailer brakes" I don't recall exactly.
We have had a lot of problems with the built-in trailer brake system on our 2500HD Sierra ever since we first bought the truck. I think they are now resolved after numerous visits to the dealer. I have read on GMC forums that many other owners have had issues too.
In my case, I was getting so fed up wth it, that I was about to throw in the towel and install an aftermarket controller for peace of mind!
My problem was erratic. All would be fine for days or weeks, then suddenly I would start getting the " Check trailer brake system" on the dash. Because was erratic and often did not throw any codes the dealer could read, it was hard to diagnose. Sometmes I would get the message with or without the trailer even being attached!
I had verified the brakes on the trailer itself and they seemed fine.
Eventually the problem was resolved when the dealer installed a new brake control module. It is located on the truck frame near the right side rear wheel. There is also a separate brake control relay back there too.
Of course my truck is a few years older and a different model so it may not be identical. In fact I think mine (2008) might be the first year GM installed the system.
Since I had so much trouble, I have accumulated some test devices I can use in future!
(1) I have a clamp meter and made up a short plug and socket cable so I can easily measure current flowing from the truck to the trailer when i apply the truck brakes - should be 12 amps + at max braking if all is well.
(2) I had four old worn brake magnets in the garage waiting to be thrown out !
Instead of throwing them out I wired them all in parallel and duct taped them together in a block, then attached them to the two proper pins on a spare trailer plug.
Any time I have doubts about the trailer brake system on the truck itself and would like to check it, I can plug the dummy magnets into the truck to simulate a known "good" trailer, and again I can verify that I have at least 12 amps flowing with my clamp meter.
I have found that even with everything working properly, I usually need to have the gain set at max (10) on the truck to get a decent amount of braking from the trailer. Even then I don't think I could ever lock up the trailer wheels.
With my last truck (aftermarket controller) and our previous SOB trailer, it was all too easy to lock up the trailer brakes if I had the gain set too high, so I found performance with the current AS setup disappointing. But the previous trailer was about half the weight of our AS so maybe that explains it.
Be interested to hear how things get resolved. You might want to do a google search on GMC trailer brake system problems - I thin you'll find lots!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.