I have a 2012 GMC Yukon with a trailer brake controller that has worked fine for the past 5 years. Now all of a sudden, my trailer brakes won't operate. I tested the voltage between ground and the trailer brakes at the connection plug at the rear of the truck, and even with no braking applied, there is a constant voltage that pulses between 3 and 3.5 volts. Not sure what this means, but if anyone can tell me what I should try next to diagnose this issue, it would be much appreciated. Trying to understand if their is an issue with the truck itself, or the trailer.
Update: This is a built-in GM brake controller. I will test the trailer brakes my applying 12V directly to the trailer plug. Won't get to it for a week or so. There is a manual lever. Neither the truck brakes or the manual lever are working.