2021 GMC Sierra 3500 Problems and Top Complaints – Is Your Car A Lemon?

Fuel system and brake issues are among the top complaints received by the NHTSA from vehicle owners

Updated on Author: Brian Jones | Editor: Rita Roberts | Reviewer: Sergei Lemberg

Driving a heavy-duty pickup truck doesn’t just make the driver feel like a boss, it helps get the toughest jobs done. The 2021 GMC Sierra 3500 is a big, bold heavy-duty pickup truck. There’s no doubt that it’s good-looking and versatile. On top of that, GMC completely revamped the 2021 Sierra model. But owners with brake, powertrain, fuel system, DEF tank, and other issues are less than impressed.  

Click on other model year to view more problems: 2018   2020   2022

Most Common Problems

Owners most frequently report complaints to the National Highway Traffic Safety Administration (NHTSA) about the powertrain and brakes of the 2021 GMC Sierra 3500. They are followed by engine and fuel system complaints. Other commonly mentioned components and features in complaints include the electrical system, structure, and tires.

Additionally, there are three recalls that affect the 2021 Sierra 3500. One of them is an urgent recall due to its fire risk. NHTSA Campaign Number 21V758000 warns that certain Chevrolet and GMC vehicles, including the 2021-2022 GMC Sierra 3500, may be fitted with a brake pressure modulator valve (BPMV) assembly that could cause an electrical short circuit because they were not tightened properly. Two sub-assemblies may not be sealed because of the bad workmanship. This is extremely dangerous because water could get into the unit.

The NHTSA warns owners with affected vehicles to park outside and away from structures until they repair the recalled vehicle.

The other two recalls both concern the Sierra’s seat belts. NHTSA Campaign Number 20V792000 affects more than half a million (624,216) GMC and Chevrolet vehicles. The issue is that the seat belt brackets may not be secured to the seat frame in the front-row center seating position. This could result in occupants not being properly restrained in the event of a crash, which increases the risk of injury.

The second seat belt recall affects 470 Cadillac, Chevrolet, and GMC vehicles. Incorrect bolts may have been used at one or more fastening locations to secure the seat belts to the vehicle. If an incorrect bolt was used, the seat belt may not be properly attached to the vehicle, and the seat belt may not restrain an occupant in the event of a crash.

Investigation into Airbags

Opened on April 9, 2021 and closed on August 24, 2023, the airbag investigation was launched because of an airbag malfunction indicator light (MIL) illumination and potential for frontal airbag non-deployment in several Cadillac, Chevrolet, and GMC vehicles manufactured by General Motors (GM). It appeared that rust particles caused the malfunction, and investigators were concerned it could result in drivers’ airbags not deploying in the event of a frontal collision. Very few inspected vehicles seemed to be affected, and there were no reported issues relating to the problem.

Ultimately, the NHTSA’s Office of Defects Investigations closed the investigation due to a “low potential hazard to drivers.” However, they stated that this did not mean a safety-related defect didn’t exist. They reserved “the right to take additional action if warranted by new circumstances.”

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2021 GMC Sierra 3500 Complaint Summary

Complaint CategoryNumber of Complaints
Service Brakes
Power Train
Fuel/propulsion System
Electrical System

Fuel System Problems

Complaints to the NHTSA and commentary in several forums discuss the ongoing fuel system issue that affects the 2021 GMC Sierra 3500 and other GM vehicles.

An owner from Colorado states in an NHTSA complaint that the “diesel exhaust fluid heater is a faulty component in the system.” However, there is a backorder on the part with no ETA. The DEF (diesel exhaust fluid) light and the check engine light are illuminated on the dash. He states that, “when you diagnose the error code it comes up with ‘P20C1 Reductant Heater C Control Circuit/OPEN’.”

An owner from Idaho states in a complaint dated April 3, 2022, that there are issues with the diesel exhaust fluid tank assembly, heater, pump, reservoir, and the connector. For example, the heater turns high, or the heater won’t work. Eventually, after several weeks, the dealership told him that “there were 11,000 parts already spoken for and 6,000 parts on backorder.” They did not have an ETA on when the parts would be available. They told the owner that they would “cover one month’s truck payment and would like me to send in my loan payment statement.” The dealer also said that GM was working on a re-designed part. “I asked why this wasn’t on a recall and he said no because it would bring people in that don’t need it replaced because their part is still working, and that GM doesn’t have a re-designed part to fix the issue.“

Technical service bulletins (TSBs) on file with the NHTSA show that GM has been aware of fuel-related problems for some time.

Examples of TSBs and Bulletins

A bulletin issued in August 2021 concerns various Cadillac, Chevrolet, and GMC vehicles, including 2019-2021 Sierra 3500 models. The bulletin affects owners from all over the world, including North America. It highlights the issue that the DEF pump cannot build pressure after replacement. It tells dealerships to only perform the procedure described in the TSB if the system doesn’t build pressure (prime) after they have replaced the DEF pump.

An urgent communication issued to dealerships on January 20, 2022, states that GM was releasing a new service update that involved 38,000 2020-2021 Chevrolet Silverado and 2020-2021 GMC Sierra vehicles including the Sierra 3500 HD. All contained 6.6 liter diesel engines. This time, the solution aimed to address vehicles with diesel exhaust fluid (DEF) tank harness connector terminals that were overheating/arcing.

There are a total of 37 fuel system, diesel manufacturer communications on file with the NHTSA under the 2021 GMC Sierra 3500. In addition to TSBs, they include several informational bulletins for dealerships and their technical staff that relate specifically to DEF issues:

  • Three provide information on DEF and the proper use of the DEF system. (September 14, 2002, March 16, 2023, and August 1, 2023)
  • Two explain why diesel engines are using more DEF due to new emission standards. (September 12, 2022, and August 1, 2023)
  • One explains how the regen works for the DEF system and how it works at different levels. (September 13, 2022)
  • Two explain how to identify possible DEF contamination. (October 27, 2020, and September 12, 2022)
  • One provides a test procedure that they should follow for a possible faulty DEF heater. (January 13, 2021)

What Forums Say

The owner of a 2021 3500 Duramax with 20K told the Duramax Forum in March 2022 that his vehicle “randomly gives me the ‘DEF tank empty’ and threatens to reduce my speed. It usually clears itself after a few miles but did go into limp mode once.” His local dealer told him that he needed a new DEF tank. However, after waiting for three months for a new one, he found out that there was a supply problem.

Other owners confirmed that they had been experiencing the same problem — but not only 2021 owners of 3500 trucks. In January 2023, a forum member stated that his top-of-the-range 2020 GMC 2500 HD Denali had an “emissions system error.” The dealership said it was because “the pins on the wiring harness and tank had burnt out.” But, again, the parts were on national backorder and it took more than a month to have the truck repaired. Less than a year later, the issue recurred. But the same dealership said they weren’t able to order the parts until they had “diagnosed” the problem. They eventually agreed to do a “special order” if he paid “a $365 restocking fee” if the repair didn’t fix the problem. He refused because he had experienced the same issue on a 2016 2500 HD Sierra in 2017.

“So GMC has had the same issue with the tanks and wiring but continues to sell the trucks with KNOWN defective parts!”

Brake Problems

An owner from Colorado states that, after he had a recall repaired at a dealership, he took delivery of what was supposedly a brand-new unit. However, he soon discovered that “the brakes are extremely soft; to the point where I must depress the pedal to the floor to get it stopped even at slow speeds. I hoped it was just an air bubble from the repair. But the fluid levels are full and the dealer had no ideas on what could be wrong at this point. I have a great relationship with this dealer, and we’ll continue working on it. But GM is releasing absolute trash lately and I fear that some poor family will load up, trusting these vehicles, and be badly injured or killed. After 200 miles of driving it back to my residence, both at highway speeds and residential road speeds, these brakes have not improved.”

In another complaint listed as a powertrain and engine problem, a Californian owner also states that the brakes are soft. They won’t stop “unless you put both feet on the brake pedal down to the floor.” He also describes how the vehicle’s throttle body sticks wide open once the engine gets to engine temperature. On top of that, he states that the transmission does not go into the correct gear.

Your Lemon Law Legal Rights

If you think you have bought a lemon, you don’t have to accept your fate. The law makes Honda pay lemon law legal fees, so Lemberg Law will assess your problems and advise you free of charge. All you have to do is call our Helpline or fill out a contact form to request a free evaluation of your problems.

Brian Jones

About the Author:

Brian Jones spent more than 30 years working as an ASE Certified Master Tech and Parts Specialist at multiple dealerships. Brian has become an authority in the industry, traveling across the country to consult for car dealerships and contributing his expertise as a writer for several major automotive publications. In his spare time, Brian enjoys working on pickup trucks, muscle cars, Jeeps and anything related to motorsports.

See more posts from Brian Jones

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