2024 GMC Sierra 2500

The contact owns a 2024 GMC Sierra 2500. The contact stated that transmission fluid was leaking onto the garage floor. The contact stated that while driving at an undisclosed speed, the transmission hesitated before shifting. The contact stated that the check transmission warning light was illuminated. The vehicle was taken to a local dealer; however, the dealer was unable to diagnose the vehicle. The vehicle was not repaired. The contact had inquired if the transmission needed to be replaced, and was informed that transmissions were on a national back order. The manufacturer was not informed of the failure. The failure mileage was 8,808.

Problem #1: Trailer brakes failed while pulling 40' fifth wheel trailer. Was driving at about 50MPH on a 55MPH road and was approaching a stop light. I went to apply my truck and trailer brakes simultaneously and quickly found out I had "lost" my trailer brake. This happened another two times within the next 10 mile trip to my house. I immediately disconnected my trailer and took my truck to the shop. The GMC Dealer fixed this issue, however, it is both scary and extremely dangerous to the public when something like this happens on a brand new truck. Problem #2: Right around 30,000 miles I was driving (unloaded) down the road at about 35MPH when I felt a hard clunk in the transmission followed by grinding noises - immediately a notice flashed on the dash stating the truck went into reduced acceleration and the check engine light came on. I brought my truck to a local GMC Dealer to be serviced on June 19th and they advised me it was the Allison transmission valve body. The part has been on back-order and my truck is still at the dealership sitting in the back lot with at least 4 other 2024 HD models. I called GM twice to inquire about Safety Recall N242454440 (covering 2020-2022 GM Vehicles) and recommend that it get extended beyond 2022 since the problem with the transmission valve body has not been resolved yet. I have not heard anything back from them since that call back in June.

Code P0777 – "Pressure Control Solenoid 'B' Stuck On" – Reduced Propulsion / Reduced Engine Power Initiated While merging onto the interstate, this code triggered a reduction in engine power, limiting my speed to 35 mph on a highway with a posted limit between 40-70 mph. Due to the limited power, I was forced to drive on the shoulder to reach the next exit and pull into a service station. I verified the code using an OBD-II reader at AutoZone. Subsequently, I had the truck towed approximately 1 hour and 30 minutes back home. The vehicle has been at a GM dealership since June 24th, where the faulty valve body has been confirmed. However, there is currently no estimated arrival date for the replacement part, which was ordered on the same date. To my knowledge, the part has only been diagnosed electronically and has not yet been visually inspected. The dealer plans to remove the valve body and transmission pan once the replacement part arrives. Notably, there were no prior warnings, messages, or symptoms before this issue occurred; this code and power reduction were the first indications of a problem.

The contact owns a 2024 Chevrolet Sierra 2500. The contact stated that while his son was driving 60-62 MPH in tenth gear, the transmission unexpectedly downshifted to seventh gear. There was an abnormally loud clunking sound coming from the vehicle. There were no warning lights illuminated. The vehicle was taken to the local dealer, where it was diagnosed and determined that the valve body needed to be replaced. The vehicle was not repaired due to the parts being on backorder. The manufacturer was contacted, but no additional assistance was provided. The failure mileage was approximately 68,000.

The contact owns a 2024 GMC Sierra 2500. The contact stated that while driving 60 MPH, the vehicle went into LIMP Mode. The contact stated that the vehicle then inadvertently lost motive power and was unable to accelerate above 40 MPH. The vehicle returned to normal functionality, but the failure became a regular occurrence. The check engine warning light was illuminated. The vehicle was taken to the dealer, who diagnosed failures with the transmission solenoid and transmission control module (TCM). The vehicle was not repaired. The manufacturer was notified of the failure, a case was opened, and the contact was referred to the NHTSA Hotline for assistance. The failure mileage was 50,000.

2024 GMC DENALI HD with approx. 12, 500 miles: The truck is constantly going into limp mode. Driving down the road, first it feels like the wheels lock up with a loud noise. Immediately goes into limp mode and check engine light comes on. Took it to the dealer yesterday 6/3/25 for repairs. I just spoke to my service advisor this morning where I was told it was the Valve Body of the transmission that needed to be replaced. However, they were on backorder and have no ETA on when they will get a replacement.