Trailer lights/ Brake issues 2020 Sierra 2500HD AT4

Kyle Mastropasqua · September 16

I have had issues with trailer lights since I purchased the truck new. This summer I decided to really dig in. I replaced the exterior trailer receptacle, the trailer light / brake wiring to the receptacle, and the trailer lamp control module. Fuses are not blown. After replacing all parts truck was giving me a trailer brake connected fault randomly, and check trailer lights fault consistently. Now it gives me service trailer brake system on start up. Dealership is indicating that faults for all lights and trailer brake are showing. They only thing that has not been replaced with OEM parts, is the harness itself.

What am I looking at the finally remedy this problem? Its just aggravating at this point, and borderline undrivable with a trailer due to the constant messages that are popping up while driving.

gmatx · September 16

What have you checked on your trailer? I had some faults pop up when I had some water intrude into one of the LED brake lights on my travel trailer. It would happen off and on as the water sloshed around inside the housing. It went away once I replaced the brake light.

Also have you tried using a 7 pin trailer brake tester?

64BAwagon · September 16

A lot depends on the circumstances when the faults occur. If its only when the trailer is connected you need to diagnose the trailer. The first thing I would do is replace the plug on the trailer side with a Pollack brand since thats who makes the ones on the trucks.

Some info on the trailer would also help direct the diagnosis.

Like @gmatx says. Find a 7 way tester (not expensive) and double check the truck. These trucks are sensitive to the janky wiring on some of the trailers. It might also be helpful to check your truck with a different trailer and your trailer with another truck.