The contact owns a 2019 GMC Sierra 1500. The contact stated that while driving at an undisclosed speed, the transmission unintentionally downshifted into first gear. The contact stated that several unknown warning lights were illuminated. The vehicle was towed to a dealer, and it was diagnosed and determined that the torque converter needed to be replaced. The vehicle was repaired; however, the failure persisted. The vehicle was towed back to the dealer, where it was diagnosed and determined that the engine needed to be replaced. The vehicle was repaired; however, the failure persisted. The manufacturer was notified of the failure. The failure mileage was approximately 86,000.