general problems of the 2020 GMC Sierra 1500 - part 1

Truck broke down with a rear-wheel lockup and had the transmission replaced in June 2025. In December 2025 and February 2026, truck lost power two times and entered limp mode while on a major highway and major interstate. The problem was misdiagnosed in June 2025, diagnosed properly in December 2025, but not properly repaired, and diagnosed properly in February 2026. All three times the vehicle had this issue, two of the same exact dtcs were present. This is a safety defect, and I have lost all confidence that the vehicle is reliable enough and safe enough for myself and my family.

I was driving the truck on the freeway and the engine completely shut off with no warning. I pulled to the side of the shoulder and I could smell a burning smell. The truck wouldn’t start back up or move in neutral. Towed it to a shop and they said it had a camshaft issue and a slight knocking noise when they got it to fire up only would run for a little then shut back off. Now I’m having to replace the whole engine due to being bad with the cam code and knocking.

The morning of December 12th, 2025 I used my vehicle to transport my daughter approximately 5 miles round trip from our home to drop her off at daycare and then I returned home. I used my front and rear defrost elements to expedite removal of heavy condensation which had formed on all windows overnight. This was approximately 7 am. I parked my truck in the driveway and went inside until approximately 11:45 am when I went to leave in my truck and noticed the rear sliding window had shattered. The window shattered and distributed tempered glass fragments all over the inside of the cab. Only approximately 10% of the window remained intact. I then reviewed the security camera footage from my driveway and discovered that the window failed entirely on its own with nothing hitting/striking/touching the window, nor any humans or animals in the vicinity. I attempted to take the vehicle to a GMC service department this morning (Dec 13) but due to their demand to keep the vehicle for 2-3 days to investigate yet unable to provide a loaner vehicle, I was unable to leave the vehicle in their possession and returned home. I plan to continue to contact GMC dealership service departments in hope I can find one which can provide a loaner vehicle, as I will need transportation to and from work in the coming days. I do have video footage of the incident although I am unable to add it below.

Coolant control valve and engine cooling fan.

Was driving down the freeway around 65 miles an hour attempted to pass a truck when the steering started wobbling the abs that ems and the breaking system came on with the check engine light almost had no control and barely avoided having an accident the only way it went back to normal was slowing down.

Power steering works sometimes and then stops trailer break system issues.

1. The vehicle’s emissions system is malfunctioning, specifically the ecm (engine control module) related to the exhaust emissions system. The diagnostic trouble code is p2c7a, indicating the system is not performing as expected. Key components such as the nox catalytic converters and diesel particulate filter have been repeatedly replaced, but the issue persists. The vehicle and all replaced components are available for inspection upon request. 2. The malfunctioning emissions system has led to an exhaust leak that allowed exhaust gases to enter the vehicle’s cab. This creates a hazardous environment for both the driver and passengers, exposing them to harmful exhaust fumes. In addition to reduced engine performance, poorer fuel economy, and emissions non-compliance, this exhaust leak poses a serious safety risk by compromising the air quality inside the vehicle. Prolonged operation under these conditions could lead to adverse health effects and further mechanical failures that impact vehicle safety. 3. Yes. The dealership has diagnosed the problem multiple times. The emissions components have been replaced under warranty on two separate occasions. The dealer attributed the fault to fuel quality; however, fuel distributor debunked these claims. 4. The vehicle has been inspected extensively by the dealership with invoiced repair work replacing key emissions control components. 5. The check engine light and service engine soon (ses) light have been illuminated with the diagnostic code p2c7a since the initial identification of the problem which started 3jun24.

Loss of power steering. Loss of engine propulsion at high rates of speed. Engine would not start.

Woke up one morning and the entire back seat, entire truck flooring was soaking wet. Got it to gerber collision in el reno, oklahoma. They said that the back window had a crack in it. My insurance said it is a mechanical issue on the sliding back glass and they do not cover such issue. Also want to turn in that the transmission in this truck had to be replaced two week prior to this incident. The truck only had 118,000 miles on it. Safe to say that this 10 point GMC diesel truck is a nightmare and the entire truck is a hazard. They need to take these truck off the market.

I was about 45 miles or so from home on [xxx]. As I left a business my truck went about 100 feet and the engine stopped working . No warning. Truck was towed to a gm dealer in wilmington IL. It was determined the cause was due to the wiring from the engine ecm that was run over the top of a very sharp plastic mounting bracket for the ecm. The wire had been cut because of the way this was done at the factory. In addition the fuel pump also needed to be replaced . At the time of this, the truck had 39,140 miles on it. I had no warning this was about to happen. No check engine light went on , etc. Most of my route home is on the highway . Had this not happened on a side street , I would have been involved in an accident. I ended up paying $865. 42 plus towing for this repair I should not be paying for. The people at the gm dealer told me this has been happening on numerous gm pick up trucks. This is not a maintenance issue or an isolated case. Information redacted pursuant to the freedom of information act (foia), 5 u. S. C. 552(b)(6).

Engine light came on, performance rough at various speeds / conditions, and smelled like burning and strong urea smell. Gm maintenance identified a failed def injector / replaced.

I was driving about 45 mph when the vehicle started to brake on its own. Message center said "brake failure max speed 62 mph". I had to move the vehicle, so I continued forward. It was still trying to self-brake, I decided to stop and turn ignition off so computers could re-set but I had no brakes. I had to stand on brake pedal to get it stopped. Turned ignition off and restarted. The problem was gone, but it happened again 2 weeks later. Took to dealership where purchased and they said it needed a computed recalibration. Something I didn't know about. They charged me $200 to recalibrate the computer. My issue is the danger of not having brakes, not the $200. How many similar vehicles could this happen to?.

Back window leaks down onto electrical compents and causes mold behind rear seat, plus staining mold on headliner.

Had GMC (leominster, MA), perform the safety recall for the transmission on my sierra. This is the one for the rear wheel lockup that requires a software update. My battery was completely dead after the vehicle sat for a week. No accessori s plugged in, and it's a 6 month old battery battery tested fine and is charging now, but it appears the cpu never fully powered down after the software update, resulting in needing multiple jump starts and 3+ hours of run time to charge the battery enough to turn over on its own. I can see this being a safety issue in relation to the auto-start, because when it is being used, the battery is needed to restart the engine. This could result in a stall at an intersection, and if drivers are not alert, may collide with a stalled vehicle.

Engine light would come on for a few days and than turn itself off. This occurred for about 8/9 months; however, instructed by the gm maintenance department that if the vehicle came in and the light turned off before they got to it I would be charged for the diagnosis and no work would be performed. Engine light came on and I was able to park the vehicle / they picked it up and read it prior to putting in their lot until a tech became available. Gm maintenance was able to diagnosis that there was an issue with the egr sensor and nox catalytic converter without filter failed / both items replaced.

While driving on interstate transmission would “lock up” momentarily creating drastic drop in speed and unable to shift gears and accelerate. “d” would start flashing in dash board. Dealership said this is unrelated to current recall about rear wheels locking up from valve body failure and instead this is an electrical issue. Dealership also stated this is the 2nd truck in a week they have had come in with this issue. Almost caused accident on interstate with my family and I as a result of sudden drop in speed and unable to accelerate with multiple cars behind us.

Driving down highway at 60 mph and truck shut off. Had to try to get it over to the shoulder to off the travel lane. Dealer found wire harness chaffed at front left shock tower ecm bracket. Several wires damaged, repaired and cleared codes. This could have been much worse had I been in heavy traffic and needed to cross additional lanes of traffic to park in a safe area. I had no power steering and limited space to park the truck. I called to police to have a car dispatched to park behind me so the truck could be loaded on a tow vehicle to bring it to the dealer. This could have been worse had my wife been driving.

I have taken my vehicle to 3 seperate mechanics (GMC, aamco and independent). They all agree that my valve body inside the transmission must be replaced. The problem is that valve bodies are on a national back order and cannot be fullfilled for months, putting my transmission at risk for further failure. Reading online forums, there are many GMC owners facing this same issue. The mechanics at GMC are requesting that I change out my entire transmission, since that will be a quicker solution than waiting months for that valve body. Now, I am being told that they are having difficulty locating an original manufactured transmission. This puts me in a serious bind because my truck is my only vehicle and there are no valve bodies or transmissions to fix this issue. My truck is only a 2020 and I just surpassed 100k miles. There were no dashboard warning lights that indicated my trucks problem. Only the fact that my truck is shuddering when shifting gears and that a diagnostic tool had to find the source of the problem. This problem started about 5 months ago and has progressivly gotten worse.

Def emissions warning and engine emissions warning lights suddenly appeared. Def emissions warning went on and off multiple times in the two days before bringing to a GMC service location. Informed that the def tank heater failed, a known problem, but that they likely would not have a repair available for over a month and would not be covered by any warranty. The vehicle is now speed limited to unsafe speeds on highways while waiting for the repair.

Hi I have always taken good care of my 2020 GMC Sierra 1500 at4 6. 2 kept up with its regular and recommended maintenance. I had no warning signs on the dash. So it surprised me that while on my way to work early one morning I was getting off the highway exit and the truck lost power, my steering wheel hardened and it posed a great threat to my life and others on the road, as it suddenly decelerated in a area where people are driving at accelerated speeds. The whole truck was loosing power and shaking hard as I attempted to steer it to a safe location on the side of the road. Had a mechanic inspect it and they determined the truck ended up with a seized engine. I have since not been able to use my truck for 8 months and still currently paying monthly payments of 1000 to pay it off.

My brakes failed 2 times. One time I almost got into a horrible wreck with my daughter in the truck. "brake system failure" " max speed 43mph" I have not had any brake work or anything done. The dealer is trying to charge me to replace the whole brake system when [xxx] this says to reprogram. Still waiting on GMC to tell me if it's covered under warranty. From my own research this has been a major problem for hundreds of people if not more. I could have died because of a faulty brake system or a bad software update. I need help with this and so do the countless other 2020 gm owners. There is a recall on the 2019 for the exact problem but not the 2020s. Gm safety recall n192268090 please advise information redacted pursuant to the freedom of information act (foia), 5 u. S. C. 552(b)(6).

My truck 2020 GMC Sierra 1500 at4 denali is not working properly since July 2024. I took it to 2 different dealership to get a diagnostic and I finally ended doing the repairs for brake issues. I have been towed 6 or more times and almost experienced 2 car accidents since I drive the truck after the supposedly repairs where done but I ended stranded again with no brakes. The dealership has repair the truck 3 times but im still getting sevaral codes the fact that I had to pay out of pocket and have to deal with this and expose myself to danger and my family in driving a vehicle that is no safety condition its absurd. I call several times the recall gm hotline and I was told GMC 1500 are included in the recalls just not the at4 and I have a family member with the same truck and same issues she ended up trading in the truck recently since she kept experiencing the same issues over and over again just like me.

Engine light came on and real bad urine smell / burning coming from the vehicle / rough performance. This issue occurred at various speeds / trips / drive types with the only variation being speed from 25mph to 75mph. Vehicle went in to discover that the reductant control module software (rcm) out of date, and two nox sensor failures.

The contact owns a 2020 GMC Sierra 1500. The contact stated that while driving 60 mph, the horn failed to function as intended. There were no warning lights illuminated. The contact stated that on a separate occasion, the right lane sensor stopped working as intended. There were no warning lights illuminated. The contact stated that on a separate occasion, while driving 60 mph in the rain or on rough asphalt, the rear suspension caused the vehicle to fishtail to the right. The contact turned to the left to control the vehicle. The contact stated that the vehicle returned to normal functionality after the failure. The low transmission fluid warning light was illuminated. The vehicle was taken to an independent mechanic, who diagnosed the vehicle with an electrical systems failure. The independent mechanic informed the contact that there was adequate transmission fluid inside the transmission, and it was unclear why the warning light was illuminated. The contact was informed that weights were added to the wheels to balance each wheel. The vehicle was not yet repaired for the other failures. The contact stated that the transmission was replaced a year prior to the failure, and the horn had failed to function soon after the transmission replacement. The manufacturer was made aware of the failure. The failure mileage was approximately 68,000.

After repair got a few miles down the road and noticed the volt meter jumping back and forth between the white lines. Line would slowly go up to max line and then slowly drop back down to lower limit and continue to go back and forth. This was very concerning to myself and I was waiting for something to fail while driving. Got about 10 miles away and was to concerned / returned the vehicle informed by gm maintenance tech that this is what / how the vehicle is designed and that it is the operating within its parameters. I expressed that only time I noticed something like this was prior to an engine light / electrical failure; however, maintenance team expressed gms stance that it is operating within tolerance.

My wiring harness which affects all systems. I was on the interstate when out of nowhere any and all warning lights came on. The power was cut and rapidly slowed down. I also pulled into traffic and the truck just lost power. The problem was confirmed by dealership. The vehicle was inspected by dealership and wiring harness was the issue. The issues appeared out of nowhere. All the warnings lights came on at once. There has been numerous issues with this part and it’s even recognized by just.

Back window frame crack led to water intrusion which flooded electric module controlling speedometer and adaptive settings. Extremely unsafe driving with faulty electrical components unknown to me. Insurance and dealership told me this is a common GMC occurrence. Cost is near $10,000 to make truck drivable.

Electronic power steering stops working randomly which makes it nearly impossible to safely drive the vehicle. The strength needed to turn the vehicle is so great that the steering wheel elevation lock fails and the wheel slams to the lowest limit of travel. Problem started around 50,000 miles and was only a warning that went away as soon as I got in the truck again. However reappeared today and the steering went out this time. The gm dealer in mexia texas didn't know what was wrong, I am taking it to a dealer in corsicana tomorrow. This is a known problem that gm has had in the past however my vehicle has no active recall to make my truck safe free of charge. This puts myself and other drivers at risk because there would be no way to rapidly maneuver the vehicle to avoid a colision.

While driving my vehicle it began down shifting harder than normal (at various speeds and road (highway, city and dirt road)); along with a noticeable burning smell after every trip. Gm maintenance personnel discovered that the wire was chafed and arching / catching on fire underneath passenger side resulting in the burning smell and the wiring issue aligned to the transmission. Personnel took a video and provided to me as evidence of the fire / smell; however, that was on their official gm app so I was unable to download.

We are unsure what happened that caused the issue. One day the dashboard just started giving us error messages, 3 of them to be specific. One says check turn signals, another says check trailer brake light and another something else about brakes or turn signals. (we have to dismiss them all before anything on the dash is viewable). This issue is causing the blinkers to blink fast as though the blinker lights need replaced and it happens when turning right or left. The alerts continue to give us error messages every few minutes, even after we have dismissed them all. We had the truck at a shop trying to figure out the issue and his guys spent hours and replaced things he thought were causing the issue but nothing fixed it. He referred us to take it to the dealership service shop for troubleshooting so we took it to the GMC dealer where we bought the truck. GMC has already spent hours, maybe 5-7 hours at $150 per hour, trying to figure out what's wrong and have been unable to find anything wrong. The truck is still at the GMC dealership shop but I can get pictures of the errors when we get it back.

Camera always stays on even when I am driving camera still on my dashboard.

Vehicle stuck in gear. Dealership was delivering the car back to myself when about a mile out from my house when the vehicles made a sudden downshift and was stuck back in gear. Driver called and notified me that it was on its way back to the shop for further work.

Normal driving conditions vehicle decelerated and warning lights and notification appeared. Warning were ‘service ecs’ and ‘steering assist is reduced drive with care’. Shut off vehicle for 30 minutes and warnings were no longer present and vehicle drove normally. Happened again 24 hours later. Received email notifications from GMC to arrive within one day. This was a know recall of the previous model year.

The contact owns a 2020 GMC Sierra 1500. The contact stated while driving various speeds, the vehicle was stuck in gear. While driving there was an abnormal burning odor coming from the vehicle. The vehicle failed to start on several occasions. There were sparks from the passenger's side front seat. Several unknown warning lights were illuminated. The vehicle was taken to the local dealer, where it was diagnosed with electrical, transmission module, wiring harness, air alternator cooler, and exhaust system failures. The vehicle was repaired; however, the failure recurred. Additionally, while driving at undisclosed speeds, there was a failure with the vault meter. The onstar system provided an error with the antilock braking system. The manufacturer was contacted, but no assistance was provided. The failure mileage was approximately 44,000.

The contact owned a 2020 GMC Sierra 1500. The contact stated that after the vehicle was purchased and while driving at an undisclosed speed, the check engine warning light illuminated. The dealer was contacted. However, the check engine light turned off, and the dealer refused to inspect the vehicle. The contact stated that while driving at an undisclosed speed, the vehicle jerked and the check engine light illuminated. The dealer was contacted. However, the warranty had expired, and the dealer refused to inspect the vehicle. The manufacturer was contacted, and the contact was offered a five-hundred-dollar coupon. The vehicle was taken back to the dealer. However, no cause for the failure was found. The contact stated that an unknown part was replaced. The contact stated that the failure persisted. The contact stated that while turning on the engine, the contact's sister informed him that smoke was emitting from under the hood. The contact turned off the engine, opened the hood, and noticed that all of the wires were melted. The contact splashed water on the wires. The contact checked underneath the vehicle and noticed that the plastic that was burnt was leaking residue. The vehicle was left at the residence where the contact's mother lives. The next day, the contact noticed that the front windshield had shattered due to the heat from the burnt wires. The insurance company was notified and referred the caller to take the vehicle to be inspected. The vehicle was towed to a collision center. The contact was informed that the vehicle had a factory defect. The vehicle was not repaired. The manufacturer was notified of the failure. However, no assistance was provided since the warranty had expired.  the failure mileage was approximately 32,000.