2020 GMC AT4 Diesel Brakes lock up on their own. Extremely Dangerous!!!

[Laura Reller]

It was the transmission valve body. They replaced and have not had it happen since. It would only happen in tow/haul mode. The dealer found a bulletin and replaced it for us.

[Fred Reynolds]

I have a 2020 GMC Denali one ton,I have 186000 kms on it, same problems, took into dealer they said my brakes where locking up, $3900.00 later with transfer case fluid change, transmission fluid and filter change, rear differential fluid changed. They also done rear brakes and parking brakes.

Still happening, going back to dealer.

I think it is tranny sensor!

We will see, they where convinced they had it fixed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Another JR]

On 6/4/2022 at 10:36 AM, Another JR said:

My neighbor confirmed the dealer told him the second event was due to the transfer case suddenly shifting into 4 low. They do not know why it occurred. They also found some collateral damage to the transmission and repaired that.  I would have thought they’d replace every part having to do with transfer case control, but apparently they didn’t. 
 

He’s hoping for the best but if it happens again it will be a clear lemon law case under WA law and I suspect he’ll have them take it back. 

I want to clarify this report so people aren’t misled about the root cause. The first event my neighbor experienced was determined to be caused by the transmission, but it also overloaded and damaged the transfer case.
 

The second event was caused by the previous repair of the transfer case leaving contamination behind that later caused the transfer case control to select 4 low on its own while driving on the highway.  I don’t remember what component he said caused this. 

 

 

 

 

 

[Jettech1]

On 9/2/2022 at 9:34 PM, Laura Reller said:

It was the transmission valve body. They replaced and have not had it happen since. It would only happen in tow/haul mode. The dealer found a bulletin and replaced it for us.

I'm glad the fix worked.  But in all honesty, that's like the worse nightmare anyone could have while towing anything....dahhhh, need to rid my mind of this...lol...hello birdies, hello trees with no leaves, hello to the whole world...LOL..

[Hannde]

This just happened to me also!! pulling my small camper through the mountains and boom truck tires lockup and release. This happened 7 times in a three hour drive very stressful and dangerous.

Im driving a 2020 silverado 1500 3l diesel 80000km's . I'm not looking forward to what the dealership tries to say!

[Jettech1]

1 hour ago, Hannde said:

This just happened to me also!! pulling my small camper through the mountains and boom truck tires lockup and release. This happened 7 times in a three hour drive very stressful and dangerous.

Im driving a 2020 silverado 1500 3l diesel 80000km's . I'm not looking forward to what the dealership tries to say!

That is some scary stuff.  I'm glad you all are ok.  Please keep us updated as to what the fix is.

[Jennifer Stow]

I have a 2019 GMC Sierra 1500, which is my work truck. Last Friday I was pulling out onto a four-lane highway and the wheels locked up screeching to a stop , as well as the cars behind me. This happened three times in a row. I could only get it to operate in reverse without this happening and had to pull off to a side road. Ended up having it towed to the GM dealer where they have reported back that they cannot find anything wrong with it or duplicate the problem. They're asking me if I hit the parking brake by mistake! Reading through this post I'm getting some ideas on what to suggest to the dealer. If anyone else has something to offer I would love to hear it! I'm nervous to drive this truck again without coming up with some sort of resolution.

[Charmayne]

On 6/7/2021 at 5:26 PM, joeidaho said:

I have a 2020 AT4 2500 Diesel with 37600 miles on it. Just under 2 weeks ago I was hauling my boat to the lake when the back truck brakes completely locked causing the vehicles behind me to veer off in to the side lanes to avoid rear ending the boat. I thought it was the boat trailer because the smoke coming off the back tires. I went another 200 yards and the truck did it again. This time everyone was back way way behind me. I couldn't pull over due to guard rails so I puttered up to the closest turn off and looked everything over. I disconnected the boat and made sure the brake away wasn't hung up and hooked it all back up again. I pulled out and went another 1/4 of a mile and it locked up again. This time I noticed the truck flashing the N for Neutral and the truck wouldn't go forward. So I had to put it in Neutral and then back to Drive so it would move forward again. I pulled over and disconnected all the hydraulics on the boat brakes and went on my way. Great weekend boating and no problem. On my way home, about a mile from the house the brakes lock up again; again causing all the vehicles behind me to slam on their brakes and go all over the place. Now, I had already called to make an appt for the trailer to get serviced. It was Sunday so I had to limp it home. Because I disconnected every logical part of the boat brake system I assumed no more issue. When I get home and I completely voided the brakes on the trailer of all fluid and took it to the boat dealer for service. The boat is new and this was the second time I towed it so I assumed it was some kind of anomaly with the boat trailer. 

Today I was pulling a fully loaded enclosed trailer and the truck locked the brakes again going 60 on a highway and all the vehicles behind me went to the sides and hurried around me to avoid a collision. Fortunately I was only a few hundred yards from my destination so I disconnected the trailer and took the truck to the closest service center. They have me scheduled to have it inspected tomorrow. 

I'm putting this on here so everyone can have the seed planted if there is some kind of computer glitch or something that comes up once the vehicle hits a certain mile mark etc. I honestly don't know but I have a fleet of other trucks for my construction business and this is the first GMC I have ever owned. I love the power and pulling capability but I would assume not many of you owners have reached this mile mark yet. But also to see if anyone else has had a similar issue. 

I'll keep everyone posted when I know more. 

Hello. What did you find out about this issue? I have the same problem.

[Charmayne]

I have a 2021 GMC AT4 Diesel pickup. I was pulling a trailer with my side by side on it when the rear brakes locked up and skidded for a split second. This happened a total of 3 times before we got to our destination. This happened again on our way back home. We crippled it back to the house at approx 40 miles per hour. The trailer weight was well within the limits of weight, so that was not an issue. We also disconnected the trailer brakes thinking this was the problem but it still locked up. Has anybody had an issue with this? GMC said they could not find anything wrong. There is definitely something wrong! This is very dangerous!

[Jeffj520]

Same issue started today.  Also got P2C7A at the same time.  Not sure if it's related.  

 

First a heavy downshift and bang.  Then about 10 miles later, full wheel lockup.  Skid for maybe a second or two.  Trailer was swerving a bit but all was fine after a couple fast seconds. 

 

Was in town mode, pulling about 6k.

[TrouserChief]

Late to this post. However, I had this issue last year on a 2020 AT4 3.0L 1500.

First time was a quick lock-up and release, wasn't sure what happened.  Progressively got worse (worst was when I was passing a semi on a two lane road, locked up with another oncoming semi). I took it to the dealer and they couldn't replicate the issue, as I'm leaving it happened two-three times repeatedly in front of the same car, needless to say they had some serious road rage at me because it looked as if I was brake checking them. At this point, I was determined to see if I could replicate it on command. Turns out it happened nearly every time I would slow down then step on the gas, so usually a sudden downshift to increase speed. Immediately went back to the dealer and explained how to replicate it. Mechanic came back ghost faced and said he definitely could replicate it. Turns out it was the TCM (transmission control module) though my truck never threw a fault code.  Powertrain warranty covered the issue and the dealership threw in a tranny flush to make sure there weren't any shavings from the transmission seizing.

It's been 20k miles since it happened, fingers crossed this was a one time deal.

Something to note, I had zero issues until I started towing heavily. It only happened once while towing but the issue started within weeks of pulling a large camper. I was at 50k+ miles with no issues prior to that. I had towed prior to then but nothing consistent or that heavy (even though it was within specs). Could be coincidental but possibly not.

TL;DR Same locking up problem. My truck never threw any fault codes but the Transmission Control Module was the cause of the issue. Replicated by slowing down and immediately increasing speed (downshifting, passing, etc.). This was covered under the powertrain in my case.

[AmandaE]

This sounds like a failed valve body in the transmission and actually a common GM issue. It happened to my suburban. 

[redwngr]

On 5/15/2025 at 12:08 AM, AmandaE said:

This sounds like a failed valve body in the transmission and actually a common GM issue. It happened to my suburban. 

 

Nobody new that when this thread started in 2021 (and was most active thru '21, '22 and '23)

 

We now now that the transmission valve was likely the cause for everyone that reported brake issues.