2012 Gmc Sierra 1500 Transmission Problems

While driving at highway speed, your truck's engine shut down completely, causing you to lose power steering and brakes. After pulling over, the truck wouldn't restart for several attempts over about 5 minutes, then finally started and ran normally except for the check engine light being on. The issue likely involves a fuel system, ignition, or electrical problem that needs diagnostic scanning to identify the root cause.

Your transmission unexpectedly downshifted, especially between second and third gears, while driving. You also noticed the check engine light coming on and off, and the engine felt like the engine mounts weren't working properly. The problem went away temporarily after restarting the vehicle, but happened again later. The dealer couldn't find the cause and didn't make any repairs. The issue may be related to engine mount failure, transmission control problems, or an engine sensor malfunction that needs diagnostic scanning and possible replacement.