2021 GMC Sierra 2500HD: Power Train complaints
Owner-reported issue (NHTSA complaints, not a recall)
Symptoms
NHTSA has 13 owner complaints for the 2021 GMC Sierra 2500HD that list the power train component. Complaints are individual owner reports, not verified defects.
Examples:
- NHTSA complaint ODI #11768450 (filed 10/02/2026): “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…”
- NHTSA complaint ODI #11746749 (filed 06/26/2026): “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…”
- NHTSA complaint ODI #11741364 (filed 06/01/2026): “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 a…”
Likely causes
These complaints do not establish a single root cause; read the examples above and get a diagnosis.
Fix, in plain English
No single fix is established by these reports. Check your VIN for open recalls at nhtsa.gov/recalls, ask a dealer about technical service bulletins (TSBs) or extended warranty coverage, then have the system diagnosed.
DIY difficulty: Pro
Diagnosis needs a scan tool; repair or replacement needs a lift, transmission jack and the manufacturer's fluid fill/relearn procedure. (Our assessment.)
Cost
No sourced cost range for this issue. Get a written quote after diagnosis.
Typical parts (search links, no affiliate codes)
AutoZone · O'Reilly · Advance · RockAuto