*Median odometer reading when the problem happened, where owners gave one. One complaint can name several problem areas.
Power train › Automatic transmission › Fluid/lubricant › Valves/valve body
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.
Sep 14, 2026 · 38,000 milesService brakes
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…
May 23, 2026Fuel/propulsion system
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.
Jul 26, 2026Power train
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…
Dec 16, 2025Power train
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.
Apr 12, 2026Structure › Body › Step/ladder
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.
Apr 23, 2026 · 93,726 milesUnknown or other
XXX Power Assist Step Reversing I have a 2021 GMC Sierra 2500HD that the local dealership (Ozark MO 65714) is stating has expired and not covered by the warranty. I researched online and as I understand this recall falls under SAFTEY recalls and is to be fixed and doesn't expire? It requires no parts but the reprogramming of the K4 Assist Step Control Module. What is my next step or recourse?
Apr 22, 2026Engine
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.
Jan 1, 2023 · 77,000 miles
Owner complaints are submitted to NHTSA by the public and are not verified. Shown as filed (NHTSA removes personal details); long reports are shortened.