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

Question

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

0

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

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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

Reply

Jameson C.

3/12/2025

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

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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

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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.

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Jeff

5/14/2025

@Buford I’ve had the same issue and am about done with GMC and Chevy. This is a known issue on these trucks to everyone but the service techs it appears. They tested mine over and over after giving me the trailer excuse. I bought 2 brand new trailers and they both did it so I called bullshit on the dealer. They swapped out the trailer connection, charging me $1200 for the convenience and it’s still an issue. I plug the same trailers into my old equipment or my neighbors Rams and Fords and they run flawlessly but the second I hit the road with my 90k truck it’s game over. My boat dealer said it’s a known issue with mostly the Denali trucks and gave me a adapter that plugs in between the 2 7 pin connections and that works for the boat trailer but the others still throw codes. What gets me is the amount of people on the forums and I talk to personally all having the same issue but all you get from the techs and dealers is deer in the headlights looks like you are the crazy one. If you can’t build it to last don’t build it at all. They throw tech in these things that is not field tested and it fails and then they lie and treat you like you’re the only one in the whole country with this issue and “it has to be the trailer”. Get F@&$ed.

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See All (4) Replies to Buford ∨

Answer 1

The Dealership is correct that the 2021 GMC Sierra is not going to send power back to the 7-way unless it senses a load. Since your trailer wiring is chewed up, the 7-way is most likely not sensing a load. The fix for this would be to use an older…

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Answer 2

I checked with Curt and you are correct, the flashing red light indicates a brake circuit overload. Since the light is flashing that most likely means you have a short somewhere on the brake output circuit drawing too much power. To correct this…

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Answer 3

You are correct, you should only be getting output when the brake is applied, or the override switch is activated. It sounds like the controller might be bad, but to make sure, you should probably bench test it before tossing it. Use a circuit…

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Answer 4

Since you are not getting the 12V power when your truck is running, you most likely don't have a fuse issue as you stated their isn't a specified fuse or relay in the owners manual. The issue is that your truck is computerized even at the…

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Answer 5

Surge brakes will activate when backing the trailer up unless you do one of two things to address the problem. You can install free-backing brake assemblies on the trailer (I attached a link to a page that has all of the ones we carry) or you can use…

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Answer 6

You said you checked the brake magnets. When inspecting them did you notice if the windings inside the magnets are showing? If so this is most likely the reason that the short is showing up on the controller when the vehicle is moving. This would…

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Answer 7

Yes, dielectric grease like the Dielectric Grease for Electrical Connectors #11755 can and should absolutely be used on electrical connectors like your 4-pin trailer connector. Dielectric grease does a great job of protecting your equipment from…

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Answer 8

Dielectric grease is basically just applied in a thin coating to the connector plug. You can use #11755 or #LT37534 if you prefer a smaller amount. I've attached a video to assist. Just like the connection on the back, you can add this to the…

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Answer 9

The difference between the Class II and Class III hitches has to do with their Gross Trailer Weight (GTW), their Tongue Weight (TW), and their Hitch Receiver Size. Class II trailer hitch receivers have a 1-1/4 inch receiver and can be rated up to…

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Answer 10

A 4-Way trailer connector has the basic lighting functions only; running lights, left turn signal and brake lights, right turn signal and brake lights, and ground. A 7-Way has these functions and a 12 volt circuit, a circuit for electric trailer…

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Answer 11

The OEM prep package consists of 4 "pucks" in the bed of the truck that will allow you to drop in puck style hitches for easy installation and removal, that gives you a completely clear bed when not in use. The B&W Companion OEM 5th…

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Answer 12

In order to determine the torque rating for the lug nuts on your trailer you will need to know check out the chart I attached that shows all of the torque specs for all the trailer lugs available based on stud size, wheel material and wheel size.

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

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