EngineFuel systemPowertrain (other)
Filed Jun 26, 2026
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.
- NHTSA ID
- 11746749
- Incident
- Dec 16, 2025
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.
- NHTSA ID
- 11728457
- Incident
- Jan 1, 2023
- Mileage
- 77,000 mi
80,000 Miles, complete engine break down. 12,000 to repair
- NHTSA ID
- 11715295
- Incident
- Jan 26, 2026
EnginePowertrain (other)
Filed Dec 19, 2024
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 NHTSA 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.
- NHTSA ID
- 11631610
- Incident
- Nov 28, 2024
- Mileage
- 134,000 mi
Driver assistanceElectricalEngine
Filed Aug 15, 2024
Smog system check and Engan warning light keeps come on.
- NHTSA ID
- 11608733
- Incident
- Aug 13, 2024
The contact owns a 2021 GMC Sierra 2500. The contact stated that the vehicle was equipped with an engine block heater that he plugged into the vehicle when the weather was below 32 degrees to start the vehicle. The contact stated that he noticed that the engine block heater was causing the GFCI outlet in his residence, to experience an electrical short circuit. The dealer was notified of the failure and informed the contact that it was a known failure. An appointment was scheduled for diagnostic testing. The vehicle was not diagnosed or repaired. The contact related the failure to NHTSA Campaign Number: 19V328000 (Engine) however, the VIN was not included. The manufacturer was notified of the failure and a case was opened. The failure mileage was approximately 55,000.
- NHTSA ID
- 11574476
- Incident
- Feb 14, 2024
- Mileage
- 55,000 mi
Diesel Exhaust Fluid tank, high pressure
- NHTSA ID
- 11464852
- Incident
- Apr 26, 2022
Driver assistanceEngine
Filed Feb 28, 2021
THE TRUCK IS PARKED AT DEALERSHIP
- NHTSA ID
- 11398288
- Incident
- Feb 24, 2021
- Mileage
- 10 mi
Higher-selling and older vehicles accumulate more reports. Counts are not failure rates and are not directly comparable between vehicles that sold in very different numbers.