Troubleshooting Check Trailer Wiring Message on 2021 GMC Denali Sierra 2500 After Driving

Updated 07/02/2025 | Published 06/23/2023

Products Featured in This Question

Hook Probe Circuit Tester

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$12.11

Dielectric Grease for Electrical Connectors, 2 oz.

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$10.96

Question:

2021 gmc Denali 3qtr ton 4x4 I plug in the trailer harness the vehicle recognized the trailer but after a few feet of travel the warning message check trailer wiring

asked by: Randy

Expert Reply:

Seeing as your 2021 GMC Sierra 2500 Denali recognizes the trailer but then shows the error message once you start driving leads me to believe the issue is with your trailer. I would start by checking the 7-way connector and adding Dielectric Grease # 11755 to the connector. If the issue still continues you will likely either have a bad ground on the trailer or short in the wiring. I have linked one of our help articles below which goes in depth on 7-way wiring which will help you further understand where the wiring goes. You will want to start checking for physical damage to the wiring and inspecting the junction box.

expert reply by: Bud M

Buford

3/12/2025

Well Bud, my 2021 GMC does the same thing as the other guys truck does. The inter grated brake controller has never worked on my gooseneck (electric over hydraulic). I can hook it up to my 2000 Dodge 2500 and the brakes and lights work just fine. The Dodge has an ancient style under the dash brake controller. Now what is your opinion of the problem? Thanks

Jameson C.

3/12/2025

@Buford Have you put your integrated controller into hydraulic mode?

Buford

3/13/2025

Not sure as to how you do that, but I will look in my GMC manual and see if I can figure it out. I personally think it is a software issue that GM has in their brake controllers, but I will give your suggestion my due diligence and let you know. Thanks

Jameson C.

3/13/2025

@Buford Yeah let me know for sure what you find. I did some searching and I believe the process is: Start the truck, hold down the brake pedal, press and hold the electronic parking brake button for about 10 seconds until a light flashes, release the brake pedal, and then immediately press the parking brake button again.