The contact owns a 2020 Chevrolet Silverado 1500. The contact stated while driving 80 MPH, the transmission unexpectedly downshifted. There was no warning light illuminated. The vehicle was towed to the dealer where it was diagnosed and determined that the transmission had failed and needed to be replaced. The vehicle was not repaired. The manufacturer was made aware of the failure. The approximate failure mileage was 120,000.