2021 GMC Sierra 2500 power train problems
12 owner complaints filed with NHTSA name the power train on the 2021 GMC Sierra 2500 — 24% of all complaints for this model year.
What owners report
“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…”
filed Jun 26, 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…”
filed Jun 1, 2026
Verbatim excerpts from complaints filed with NHTSA. Reports are not independently verified.
Related recalls
- Transmission control valve may fail and cause the rear wheels to lock-upCampaign 24V797000 · Oct 24, 2024
Power Train problems in other Sierra 2500 years
| Year | Reports | Share |
|---|---|---|
| 2025 Sierra 2500 | 3 | 12% |
| 2024 Sierra 2500 | 23 | 24% |
| 2023 Sierra 2500 | 20 | 67% |
| 2022 Sierra 2500 | 5 | 12% |
| 2020 Sierra 2500 | 22 | 30% |
| 2019 Sierra 2500 | 2 | 6% |
| 2018 Sierra 2500 | 3 | 9% |
| 2017 Sierra 2500 | 6 | 16% |
| 2016 Sierra 2500 | 1 | 2% |
| 2015 Sierra 2500 | 8 | 9% |
| 2014 Sierra 2500 | 3 | 10% |
| 2013 Sierra 2500 | 5 | 11% |
Other reported problem areas on the 2021 GMC Sierra 2500
Counts are unique NHTSA complaints naming this component group. Complaints are unverified owner reports; mileage is computed only from complaints that included a usable odometer reading.