2021 GMC Sierra 2500 transmission problems

Filed Oct 2, 2026 · Transmission · 38,000 miles · GA

The contact owns a 2021 GMC Sierra 2500. The contact stated that while driving at an undisclosed speed, the transmission unexpectedly shifted out of first gear and the vehicle failed to accelerate as needed. The failure had occurred twice. The vehicle was towed to the dealer, who determined that the valve body needed to be replaced. The failure mileage was approximately 38,000.

Filed Jun 26, 2026 · Engine, Fuel system · IL

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 the federal safety agency's evaluation of similar vehicles.

Filed Jun 1, 2026 · Transmission · CO

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.

Filed Mar 24, 2026 · Transmission · NH

While driving on the highway at normal speed, my vehicle was operating completely normally with no warning lights, messages, or prior issues. Without any warning, the vehicle suddenly lost the ability to accelerate and felt as if it had been shifted into neutral. I immediately had to pull over to the side of the highway, as this created a dangerous situation with surrounding traffic. The vehicle had to be towed to a mechanic. Upon inspection, I was informed that a coolant line connected to the transmission had completely blew off, causing a rapid loss of transmission fluid and resulting in total transmission failure. I was told this was not due to a rusted or deteriorated clamp, but rather a sudden component failure. This vehicle has been meticulously maintained and I have owned it for less than one year. There were no prior symptoms, leaks, or warning signs leading up to this incident. This appears to be a sudden and unexpected mechanical failure, not caused by neglect or misuse. The loss of power while driving at highway speed posed a serious safety risk to myself and others on the road.

Filed Dec 8, 2025 · Transmission · MI

Pulling into a driveway and the truck made a loud clunk and the check engine light, traction control light, and a reduced engine power light came on and the truck came to an abrupt stop. When the gear shift was put into reverse or any forward gear the truck would rev up without touching the gas pedal. The left rear wheel locked up. After paying over $700 in tow bills to get the truck towed to our house and then to the dealer we were told that the transmission had a catastrophic internal failure. This truck only has 82,000 miles on it and is a 2021!

Filed Oct 29, 2025 · Transmission · 78,000 miles · CO · crash · 1 injury

The contact owns a 2021 GMC Sierra 2500. The contact stated that while driving at 50 MPH, the contact was pulling a trailer that he had loaded, and he drove around a curve, the gear shifted down independently, and the trailer pushed the vehicle off the right side, causing him to crash into a post and embankment. The contact mentioned he was able to shift the gear into Four Wheel Drive to get back on the road. The contact mentioned he pulled over to the right side of the road, exited the vehicle to inspect, and noticed damage to the vehicle and the trailer. The contact stated no air bag deployed. The contact stated that he sustained injuries left arm and neck, causing his muscles to tighten. No medical attention was required. The contact stated no police report was filed. The contact mentioned that the vehicle was previously repaired for a manufacturer's recall with the transmission, but failed to prevent the failure. The contact was relating the failure to the federal safety agency Campaign Number: 24V797000 (POWER TRAIN). The vehicle was serviced by the dealer, but the remedy failed to repair the vehicle. The contact stated that the failure was not diagnosed by a dealer or independent mechanic. The manufacturer was not notified of the failure. The approximate failure mileage was 78,000.

Filed Mar 29, 2025 · Transmission · SC

Radiator hose connection broke and loose fluid and transmission got broken

Filed Mar 20, 2025 · Transmission · OH

While driving down the highway pulling a trailer the truck started to slow down rapidly. We were able to pull to the side of the road before the truck came to a complete stop on its own. From that point forward, we could not get the truck to move forward even in 4WD. Ended up towing the truck back and a repair shop determined the transmission needed replaced. About 4 months later, we received notice of a recall describing a similar scenario. I submitted the invoice and documentation to GM, but my claim was denied because the codes and symptoms did not match the criteria for the recall.

Filed Mar 13, 2025 · Transmission · KY

Hard shifting. Back wheels locking up

Filed Dec 19, 2024 · Engine, Transmission · 134,000 miles · FL

The contact owns a 2021 GMC Sierra 2500. The contact stated while driving 40-55 MPH, the vehicle started losing motive power. The contact stated while pulling into a nearby independent mechanic shop, the vehicle started jerking significantly and stalled midway. The check engine warning light was illuminated. The mechanic diagnosed the vehicle with transmission failure. The vehicle was towed to the residence. The local dealer and several unknown dealers were notified of the failure and informed the contact that the transmission assembly was not available and could not be ordered. The vehicle was towed to CJ's Transmission, where the transmission was taken apart. The mechanic determined that the transmission needed to be replaced. The vehicle was being repaired with a rebuilt transmission. The contact received notification of the federal safety agency Campaign Number: 24V797000 (Power Train); however, the part to do the recall repair was not yet available. The local dealer was contacted. The contact stated that the manufacturer had exceeded a reasonable amount of time for the recall repair. The manufacturer was made aware of the failure and offered a modest monetary compensation for the repair. The failure mileage was approximately 134,000. Parts distribution disconnect.