Watch the engine — first year of the big 6.2L failure recall, which hits Denali trims hardest.

If it's the 5.3L, the lifter issue rides on. DFM can deactivate any of all eight cylinders, and mechanics say its lifters fail more often and earlier than the older AFM — a tick, misfire, power loss, and a $3,000–$10,000 repair with no recall, only a class action and reactive warranty repairs. A DFM disabler (~$150) is cheap prevention and a cold-start tick is the walk-away sign. The 6.2L's risk is largely covered by its recall while the 5.3L's lifter risk is not — factor that into which one you buy.

The 8-speed can still shudder and shift harshly, but the expensive control-valve failure is covered on this year by GM's 15-year/150,000-mile Special Coverage (from the in-service date, regardless of ownership) — a new valve body free, with reimbursement for prior repairs. The 10-speed on other trims is the stronger unit. Test-drive for shudder and confirm the coverage window. There is a second 2021 transmission program worth checking alongside it: a Customer Satisfaction Program for trucks with the 8-speed (GM's option code MQE) that “may have a broken reaction carrier in the transmission,” where the remedy is to replace the transmission. It is the first federal document tying a 2021 truck's 8-speed to a full replacement rather than a valve body or a fluid change — VIN-check it as well as the control-valve coverage.

GM ran a Customer Satisfaction Program for 2021 trucks with a gas engine whose water-pump circuit board "may have been manufactured incorrectly potentially causing a water pump failure." What makes this one worth knowing is how much warning GM says you get: rising coolant temperature triggers repeated messages, then the check-engine light with trouble code P2B85, then a high reading on the oil-pressure gauge and reduced power, then a chime, the air conditioning shutting itself off, and finally a message telling the driver to idle. That is a long runway before damage — but only for a driver who acts on it. The program does not name a specific engine, so it can't be pinned to one. Ask whether the pump has been replaced, and treat any coolant-temperature warning on this truck as urgent rather than informational.

A Special Coverage covers trucks whose evaporative emissions purge pump can fail, which turns on the check-engine light and sets a trouble code; GM's wording is that "the repairs will be made at no charge to the customer." It is an emissions component rather than a driveability one, so in practice it means a lit dash and a failed emissions test rather than a breakdown. The document states no limit in years or miles, so have the dealer confirm the terms from the bulletin before counting on it.

Two things to check on a 2021 Sierra. The power sliding rear window (2019–2022 crew cabs) can leak where the sliding pane meets the frame, wetting the headliner and window electronics; GM replaced windows under warranty but leaks can return. And the Sierra-exclusive MultiPro powered tailgate has drawn reports of abrupt operation — exercise every function. Check the rear headliner and floor for staining and musty smell.

GM ran a Customer Satisfaction Program for certain 2021 trucks whose eBoost brake unit assemblies could develop brake-function issues over time, replacing the unit free. It's a targeted program rather than a widespread failure — worth a VIN check to see whether it applied and was done.