2021 GMC Sierra 2500 Problems

I am reporting repeated failures involving both the transmission and diesel emissions (DEF) system on my 2021 GMC Sierra 2500HD Denali equipped with the 6.6L Duramax diesel engine and Allison 10-speed automatic transmission. While under warranty, the truck required replacement of the aftertreatment fuel injector and later the Reductant Control Module due to DEF/emissions system failures. General Motors also completed recall-related transmission software updates on the vehicle. At approximately 154,833 miles, the transmission began shifting improperly, set transmission fault codes, and required complete replacement at my expense. Less than 5,000 miles later, the vehicle again experienced a "Service DEF System" warning with multiple diagnostic trouble codes. Repairs required replacement of the DEF tank, DEF heater assembly, and NOx sensor. These repeated failures involve major drivetrain and emissions components on a heavy-duty truck. DEF system failures can place the vehicle into reduced-power (derate) mode, limiting vehicle performance and creating potential safety concerns while driving or towing. I am submitting this complaint so these failures can be documented as part of NHTSA's evaluation of similar vehicles.

We keep getting an error message that shows we need to service DEF SYSTEM, it also started to limit my MPH and is now locked at 55 MPH. initial check of the DEF SYSTEM shows a malfunctioning module. it has been happening over the last year to 18 months. they would reset it or it would sometimes go away on its own after having to drive it in order for the error to clear.

My right headlamp/ turn signal assembly began flashing at random intervals. The high beam would come on and off randomly. The entire assembly would randomly flash along with the turn signal. The headlamp would go on and off randomly. This included switching between setting. High beam, low beam, daytime running light, and completely off with no warning. This causes blinding conditions for oncoming drivers, loss of visibility for driver, and loss of visibility during inclement weather due to high beam coming on and off, then entire unit turning off. As of right now, the vehicle is undrivable due to incorrect lighting, and flashing lights.

Known issue and has been attributed to a manufacturing defct, but no recall- Leaking headlight caused damage to headlight- now turn signal and headlight do not work. Entire assembly needs to be replaced, GM / Extended Warranty Company will not cover, this safety part. Head light and turn signal were working on the way to the dealer, 1 month later when we picked it up - Right turn signal malfuction reading appeared on dash, headlight blinking and will not work. Picked up on Friday 1/30/25, and made an appointment for the following Wednesday to take it back in. Was told by dealer today that the replacement will not be covered and will cost $2100.00 to replace. This is a safety part and the truck can not be driven legally or safely without both headlights working.

I am reporting repeated failures involving both the transmission and diesel emissions (DEF) system on my 2021 GMC Sierra 2500HD Denali equipped with the 6.6L Duramax diesel engine and Allison 10-speed automatic transmission. While under warranty, the truck required replacement of the aftertreatment fuel injector and later the Reductant Control Module due to DEF/emissions system failures. General Motors also completed recall-related transmission software updates on the vehicle. At approximately 154,833 miles, the transmission began shifting improperly, set transmission fault codes, and required complete replacement at my expense. Less than 5,000 miles later, the vehicle again experienced a "Service DEF System" warning with multiple diagnostic trouble codes. Repairs required replacement of the DEF tank, DEF heater assembly, and NOx sensor. These repeated failures involve major drivetrain and emissions components on a heavy-duty truck. DEF system failures can place the vehicle into reduced-power (derate) mode, limiting vehicle performance and creating potential safety concerns while driving or towing. I am submitting this complaint so these failures can be documented as part of NHTSA's evaluation of similar vehicles.

The contact owns a 2021 GMC Sierra 2500. The contact stated that while driving at 55 MPH, the vehicle stalled unexpectedly. There were no warning lights illuminated. The vehicle was taken to the local dealer, where it was diagnosed, and determined that the engine needed to be replaced. The engine was replaced; however, the failure recurred. While at a stop and attempting to drive around a curve, the vehicle stalled. No warning lights were illuminated. The vehicle was taken to the local dealer, where it was diagnosed with engine failure. The vehicle was not repaired. The failure mileage was approximately 77,000. The VIN was not available.

The left rear internal "top-hat" parking brake shoes suffered a total manufacturing bonding failure, causing the friction lining to completely delaminate and detach from the metal backing plates inside the rotor hub. This triggered Diagnostic Trouble Code (DTC) C060B (Left Park Brake Actuator Circuit). The failed components were replaced by the servicing facility and are unavailable, but the official dealer diagnostic scan sheet and itemized repair invoice are preserved and available for inspection upon request. Safety was compromised in two ways. First, the mechanical failure completely disabled the electronic parking brake system, forcing a heavy-duty truck to rely exclusively on the transmission's parking pawl to remain stationary. This creates a severe rolling vehicle hazard, especially on inclines or when hauling/towing heavy loads. Second, when these shoes disintegrate, the loose friction debris trapped inside the enclosed drum can shift, wedge, and cause a sudden mechanical wheel lockup or severe brake dragging at highway speeds, threatening vehicle control. The problem was officially confirmed and diagnosed by an authorized General Motors dealership service center. The technician scanned the vehicle, retrieved DTC code C060B, and confirmed upon inspection that the internal parking brake shoes had completely disintegrated. There were absolutely no physical symptoms, abnormal noises, dragging sensations, or brake pedal changes prior to the failure, as the parking brake shoes sit isolated inside the rotor hub. The issue appeared suddenly on May 23, 2026, when the vehicle immediately displayed a parking brake malfunction warning lamp on the instrument cluster dashboard and completely refused to engage the electronic parking brake.

2021 GMC Sierra 2500HD 6.6L gas 6 speed automatic transmission W/4wd After a brake service job the vehicle will randomly kick on the traction control/ABS and the right front brake will lock up. The steering wheel jerk is violent enough to remove the wheel from the drivers hands. No Traction control or ABS lights flash on the dash when this occurs and it has caused the vehicle to veer into oncoming traffic and also off the road. The dealer was unable to recreate the issue nor are there any stored codes/issues found when having the truck diagnosed. ABS and speed sensor wires have been reviewed and none were damaged during the brake job. All wheel speed sensors are functional and reading when the issue happens. This issue happens during dry or wet conditions and is random without warning.

When I’m in reverse the up the cameras turn off. On one incident I was backing up and the camera turned off causing me to back up into the wall of my house

Headlights and turn signals failed. Flickering of various lights (Daytime running lamp, low beam, high beam, turn signals) followed by total failure. 3700 dollar repair at dealer to replace both headlamp assemblies. Barely over miles of warranty, but still in the 3 year warranty time period. Not having forward lighting is of course a major safety issue. Vehicle should be recalled. Many others are having the issue.

The contact owns a 2021 GMC Sierra 2500. The contact stated that on several occasions, the automatic power side-out step failed to extend as designed. The vehicle was not diagnosed or repaired. The manufacturer was notified of the failure. The failure mileage was 93,726.

My tailgate unexpectedly open as I was driving down the road. (2021 GMC 2500 Denali Sierra) This was the second time this has happened and lost things driving down the road. (3 propane tanks and a small grill the first and then an air compressor the second time) Both times it happened I opened and closed the tailgate with the vehicle running. Both times I was unaware of tailgate opening and did not notice until I got home. The first time it happened I thought it was operator error but after this weekend, I looked into this and see there was a recall on it this month. I still felt it was necessary to report incidents.

Known issue and has been attributed to a manufacturing defct, but no recall- Leaking headlight caused damage to headlight- now turn signal and headlight do not work. Entire assembly needs to be replaced, GM / Extended Warranty Company will not cover, this safety part. Head light and turn signal were working on the way to the dealer, 1 month later when we picked it up - Right turn signal malfuction reading appeared on dash, headlight blinking and will not work. Picked up on Friday 1/30/25, and made an appointment for the following Wednesday to take it back in. Was told by dealer today that the replacement will not be covered and will cost $2100.00 to replace. This is a safety part and the truck can not be driven legally or safely without both headlights working.

I purchased a 2021 GMC Denali 2500 in December 2023. A couple days after purchasing the vehicle I received a message on the dash saying the front camera was not working. I took the vehicle into the dealership where they said they reset the computer and nothing was wrong with the vehicle. One month later I was involved in a collision where a vehicle turned in front of my vehicle and the Automatic Emergency Braking failed to activate. I applied the brakes but due to my delayed reaction time 1.5 seconds, I broadsided the small vehicle with my large truck. Luckily the speed was only 20 mph and no one was hurt or killed. At this time I did not know my vehicle was equipped with AEB. In October I discovered through GM and a different dealership that my vehicle was equipped with Automatic Emergency Braking. I have had several occasions when the AEB including the crash where the AEB should have activated but never did. In 11 months I have never had the AEB activate. I took the vehicle into the dealership where I bought the vehicle and they said their computer said everything was operating correctly. The tech never drove the vehicle where the AEB should have activated. I have driven many cars and even the same model vehicle as mine, so I know what the AEB feels like and my vehicle the AEB is not working. The dealership told me to take the vehicle they checked everything GM told them to check. When I talked to GM they told me to immediately take the truck to the dealership and the dealership did nothing.

The contact owns a 2021 GMC Sierra 2500. The contact received Manufacturer's Recall Number: N232426400, for water intrusion into the electronic gate-release switch. The vehicle was repaired. The contact stated that six months after the vehicle was repaired, the tailgate failed to operate as intended. While at the residence, the taillights and headlights were flashing. The contact also became aware that the tailgate had unlatched independently while the vehicle was parked. The message "Integrated Tailgate Open or Integrated Tailgate not Latched" was displayed. The contact stated that there was an abnormal clicking sound coming from the actuator when the tailgate opened independently. The vehicle was taken to the dealer, where it was diagnosed that there was water in the switch. The dealer informed the contact that the repairs were already made and, due to the water intrusion into the switch, the dealer refused to redo the repair under the previous recall. The vehicle was repaired. The manufacturer was notified of the failure, and denied assistance since the recall repair was already performed. The failure mileage was approximately 53,000.

The contact owns a 2021 GMC Sierra 2500. The contact stated that the VIN was included in NHTSA Campaign Number: 24V060000 (Latches/Locks/Linkages) and the main exterior touchpad switch assembly was replaced. The contact stated that after the recall repair the second exterior touchpad switch assembly suddenly short circuited and caused the trunk to open unintendedly. The vehicle was returned to the dealer who stated that that only one exterior touchpad switch assembly was replaced, despite the remedy stating that the exterior touchpad switch assemblies would be replaced. The vehicle was not repaired. The manufacturer was informed of the failure and opened a case. The manufacturer referred the contact to the NHTSA Hotline to report the failure. The failure mileage was 55,700.

The “Service Safety Restraint” alarm goes off continually on my 2021 GMC Sierra 2500HD AT4. The warranty is no longer valid as at the time of occurrence I was at 34,500 miles and when I took it in the warranty had expired due to exceeding 36,000 miles. Extended warranties were not purchased and this began approximately 8-9 months after purchase. The code given is “b0014 Driver Seat Side Air Bag Deployment Loop” with a description of high resistance in wiring or wiring harness. After much research I have found that many GMC 2021 vehicles and previous models have had this exact same issue. No dealership as of yet has offered to help or understand that this defect was of their own flawed design. The issue that I am having is exactly the same as other models such as the 2021 Yukon, Sierra 1500, Suburban and others. This is a safety issue and only want GMC to account for their flaw. I do not believe I should have to pay for a design flaw or equipment malfunction that is well known amongst GMC vehicles. I am asking the NHTSA to help me in this matter and the many others who are having the same issue and for the manufacturer to be accountable for their own flaw.

Hit in the driver side tire, air bags didn’t release. Truck then went up in flames.

After finishing a long drive south, I shut the engine off and a yellow engine code popped on. I connected my computer to it and revealed a P129F code (fuel pump control module). I have noticed that other GMC models have been included in a recall concerning this problem. The 2021 GMC 2500 Denali should be also since it is the same problem. This needs to be covered by the manufacturer since it is a constant problem.

I am reporting repeated failures involving both the transmission and diesel emissions (DEF) system on my 2021 GMC Sierra 2500HD Denali equipped with the 6.6L Duramax diesel engine and Allison 10-speed automatic transmission. While under warranty, the truck required replacement of the aftertreatment fuel injector and later the Reductant Control Module due to DEF/emissions system failures. General Motors also completed recall-related transmission software updates on the vehicle. At approximately 154,833 miles, the transmission began shifting improperly, set transmission fault codes, and required complete replacement at my expense. Less than 5,000 miles later, the vehicle again experienced a "Service DEF System" warning with multiple diagnostic trouble codes. Repairs required replacement of the DEF tank, DEF heater assembly, and NOx sensor. These repeated failures involve major drivetrain and emissions components on a heavy-duty truck. DEF system failures can place the vehicle into reduced-power (derate) mode, limiting vehicle performance and creating potential safety concerns while driving or towing. I am submitting this complaint so these failures can be documented as part of NHTSA's evaluation of similar vehicles.

The “Service Safety Restraint” alarm goes off continually on my 2021 GMC Sierra 2500HD AT4. The warranty is no longer valid as at the time of occurrence I was at 34,500 miles and when I took it in the warranty had expired due to exceeding 36,000 miles. Extended warranties were not purchased and this began approximately 8-9 months after purchase. The code given is “b0014 Driver Seat Side Air Bag Deployment Loop” with a description of high resistance in wiring or wiring harness. After much research I have found that many GMC 2021 vehicles and previous models have had this exact same issue. No dealership as of yet has offered to help or understand that this defect was of their own flawed design. The issue that I am having is exactly the same as other models such as the 2021 Yukon, Sierra 1500, Suburban and others. This is a safety issue and only want GMC to account for their flaw. I do not believe I should have to pay for a design flaw or equipment malfunction that is well known amongst GMC vehicles. I am asking the NHTSA to help me in this matter and the many others who are having the same issue and for the manufacturer to be accountable for their own flaw.

LT245/75R17 I bought these tires on Walmart. com June 2021 and had Walmart install with road hazard and lifetime balance, I returned to Walmart in April 2022 and was told I had a defective tire, that it was coming apart, I can see the fibers around the issue area and soft spongy for approximately 14 inches sounds like glue not sticking when I push around the bead area, Walmart told me it was a manufacturer defective tire and they included that on the work order and also told me to take tire and contact manufacturer which I did, I have spoken and also emailed with a Michael Dorr with Unicorn Tire. without any positive results only denied a claim. this tire needs to be Inspected and I want a new one, this for sure is defective.