Throwing this is MISC and hoping someone sees and can point me in the right direction.

2009 GMC 2500HD 6.0 Gas 6L90
Pretty loaded SLT. 190K miles. Home made tune up (tire correction, mild shift improvement, mild fueling mods).

About a year ago I started getting random Service Trailer Brake on the dash. It wont appear occasionally - sometimes when it rained, and then disappear. The factory trailer brake worked fine. About 3 months ago it started coming on ever time.

I replaced the ITBC module, dash controller, relay - all with GM genuine parts - twice. Nothing fixes it. I looked over all the wiring, ohm'd out what I could - no smoking gun.

I still occasionally tow stuff with it so I bit the bullet and installed a Tekonsha P3. I even followed the TSB:

-unwound and added the two under hood power wires
-added a 30A J fuse to the missing stud for the trailer power line
-Ran a 12G wire from P3, along frame, all the way back to trailer plug.

This was Saturday morning. P3 seems fine (I don't have a trailer to test on handy) but I am still getting the dash message. According to the TSB it should have done away when I added the P3 as we disable it entirely.

Sunday we went from 88 and sunny to 43 and raining. Truck wont start. I get every message and light on the dash. 4WD Error, ABS, CEL - the whole fucking christmas tree. I finally get a scanner to connect and I got U0073, P0700. I check every fuse, look around, nothing.

Finally I got mad and kinda stomped around in the driver seat a little and the truck fired up. No gauges, lots of lights, but it started and ran.

Today I went through and cleaned every ground and power connection I could find. Every connection on mega fuse, ground of head (added another 12G ground to chassis from here), both the chassis grounds, the inside grounds by the steering column. Everything. I also loaded in my original bone stock map (just tire correction).

I am at a loss. The truck runs fine right now and doesn't throw codes, but I still get the Service Trailer Brake message. The last thing I want is for it to have a shit fit somewhere that isn't my driveway.

My Questions:

-Does anyone know of a way to disable the ITBC system entirely? I tried yanking module, I tried removing fuse, I tried removing the power wire that is supposed to feed the system.
-Any ideas on a ground i might have missed? Anyone had this issue and could point me in the direction I could look?


I have had this truck since 70K miles and its been a workhorse. I would like to keep it as a tow pig for many years to come but this is driving me nuts. I don't care so much about the message as I do about making it reliable.