Troubleshooting No Power to Trailer Brakes When 2013 GMC Sierra 2500 Brake is Pressed
Updated 03/12/2026 | Published 02/22/2025
Question:
I have a 2013 Diesel GMC Sierra 2500hd 4x4, I have replaced trailer brakes relay and trailer brake control module and I still don’t have an increase in voltage at the plug when I press the brake. The fuses are all good. I took the factory brake control on the dash off and I’m getting power to the controller on the blue and yellow top wires with the truck turned off. What else can it be? Also when I check the brake wire at the plug I get a very faint light but doesn’t get brighter when I press the brake.
asked by: Jesus G
Expert Reply:
Hey Jesus, thanks for reaching out!
Most brake controllers now days are proportional, like the Prodigy P2 # 90885, and they have a sensor inside that can tell if the controller/vehicle is actually in motion or not. If the vehicle isn't in motion, then it won't send power back to the brakes when the brake pedal is pressed because it technically isn't needed.
What you need to do is make sure power is going to the brakes when you use your manual override, and then hook up a small trailer and take your towing setup to an empty parking lot and check to see if the trailer brakes are powered when you're pressing the brake pedal while in motion.
Let me know what you find!
Earl
3/12/2026
Hi Jon, I hope it's ok to jump in here with a similar problem. It's been hard finding anyone that has info addressing this specific truck. I also have a 2013 Sierra 2500 HD but it's gas. I got just a single blip (flash) on my test light when testing the blue trailer brake wire at the rear of the truck. I removed the test light from the wire and then touched it again and got the same response. I then connected the blue wire to the trailer plug on the bumper and connected to the trailer. I could hear a blip, blip, blip at the trailer tires even without pressing the brake pedal. I did pull forward a little ways to see if the trailer brakes engaged and they did not when I pressed the brake pedal or the manual lever on the truck's factory brake controller. I have checked all fuses and they are good. Where do I go from here?
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Jon G.
10/30/2025
Hey Jesus, how did things go when you tested the trailer brakes with the Prodigy P2 while driving? It’d be great to hear if you saw full power to the brakes once the truck was in motion. Your results could help others troubleshooting the same issue.