2021 GMC Sierra 2500 Trailer Light Issues - "Check Trailer Rear Lamp" Message

rickmc17 · July 29, 2021

Hi! (Sorry about the long winded narrative in advance). I'm been dealing with a Trailer light issue on my 2021 Sierra 2500 HD hoping for suggestions. I plug in my 7 pin connector from the Airstream to the truck. As soon as the connection is made the marker lights on the Airstream come on. However, the truck light switch is in the "off" position. I jump in the truck and start it. I get the two messages below on the dash message center and the center stack.

Shut it off, disconnected and reconnected the 7 pin plug. Restarted, then ran the intergrated lighting check and verified that all of the Airstream and Truck brake, signal and tail lights function as normal. As soon as the trailer light check is complete, I get the messages below again. The truck shows as connected and trailer braking system is active.

The initial event took place at an Airstrean dealership 1500 miles from home. Up until this point I had been towing for 8 weeks and 3200 miles without any issues. Based on the truck's fault message, I sought out a service tech to check the Airstream wiring and male connector. I was with him as he went through the steps on the Airstream side. All Airstream connector circuits checked out. He then plugged a scanner into the Truck's 7 pin female connector. He received a "LT Lamp fault".

I thanked him plugged the trailer back into the truck and and headed to close by GMC dealership. Sure enough, the GMC service tech who offered to do a quick check got the same "LT Lamp" Fault light on his scanner while plugged into the truck's 7 pin connector. Unfortunately, the did not have a schedule opening for a few weeks. I was on my way home so I decided to head home and make an appointment at my local GMC dealer to trouble shoot the issue.

That night I decided I needed to disconnect the connector as the maker and tail lights of the airstream were still on after the truck was shut off. I did not want to kill the truck battery as I wanted to get an early start home. The next morning, I got up plugged the trailer into the truck. No marker ights. I thought, what ever issue occured the other day might have been reset knowing the truck lighting module performs some self checks. I ran the on board lighting test OK. Ready to go. Put it in gear and the trailer brakes were locked up. Unplugged and reconnceted the connector still locked up. I crawled under the truck bumper at the back of the connector looking for loose or broken wires. Nothing. Tapped on the connector, and the camper next door to me said "the lights came on". Sure enough put it in gear and off we went.

I ran 1140 miles straight through from Carlock IL to Clermont FL stopping only for gas and did not touch the connector. Parked the trailer and brought the truck to the dealership this morning to chase down the issue. Well, as luck would have it, they found nothing. No codes, no code history. The connector receiver was fine. As I was told..I drove the truck to the Airstream and plugged the connector in. Bingo there were the fault messages again.

To verify once again, the Airstream was not causing the fault, I swapped out the Sierra for my 2021 Jeep. Plugged the Airstream connector into the Jeep's 7 pin receptacle, no marker lights came on until I switched them on via the Jeep light switch. Also no connection issues all lights and signals worked.

Has anyone dealt with a similar issue or know about any TSB's that might help remidy this problem? I've got a nice trip planned next month but can't go until this issue is resolved.

Thanks in advance.

Clarence Stevens · August 24, 2021

Exact same issue (minus the locked brakes) with 21 GMC 2500HD and Airstream TT... following

Edited August 24, 2021 by Clarence Stevens

dilly03 · September 4, 2021

On 8/24/2021 at 12:23 PM, Clarence Stevens said:

Exact same issue (minus the locked brakes) with 21 GMC 2500HD and Airstream TT... following

Same here with a deck over trailer…mine is a 2020, but it seems to only throw the connection messages when the truck lights are on. As soon as I turn the headlamps/park lamps off, the connections all work fine. Surely someone on here has seen this before?

rickmc17 · September 4, 2021

It's a faulty trailer light module on the truck. GMC replaced under warranty.

4x4Hank · July 29, 2022

On 9/4/2021 at 2:49 PM, rickmc17 said:

It's a faulty trailer light module on the truck. GMC replaced under warranty.

I know that this is an old post but I wanted to thank you for posting this topic. I bought a used 2022 GMC 2500 about 6 weeks ago. The first time that I pulled our trailer I had the exact same problem. After our camping trip I brought my truck in to the dealer and they took the easy way out by saying that it must be the trailer. The trailer is brand new and I towed it with my previous truck without this problem so I knew better. I brought my truck in again and showed them this post and sure enough they came back to me in about a half hour saying that I was right, it was the trailer light module. They had a new one overnighted, installed it and problem fixed. Thank you again. This is what forums can do for users.
Hank

billdlc · July 30, 2022

Glad I found this post. I have a 2022 sierra 1500 limited AT4 that is 3 months old, one the way home after a 1300 mile trip towing our new 2022 winnebago Minie we were 30 miles from home going 65 mph and the the left front trailer wheel brake just locked up, the right rear did not lock but by the time I could pull over that brake was smoking. Now I have the exact same message and diagram on center screen as Rick had. Disconnected & reconnected and the brakes unlocked, we got home, but trailer lights are now always on. Hooked trailer to a 2022 f350 and everything is fine. Dealer ship has ordered new trailer light control module. NOW THE BIG QUESION IS : Will GMC replace my $200 Goodyear endurance trailer tire that is flat spotted down to the cord. Obviously this is a known problem.