I've had a 2009 & 2022 GMC Duramax, great trucks, no issues. I just purchased a 2025 GMC Denali 2500 HD Duramax and I am unable to shift into a higher gear when descending a hill even when in Low Manual. Tapping the break causes it to shift even lower and the engine revs even higher. I took it to the dealer service department and the diesel tech said it will hold the gear on a descent and offered no solution to controlling the high revs other than to slow down, but as I said breaking causes it to down shift and rev even higher. I'm hoping someone will be able to direct me to a company that makes a programmer or something that will allow me to manually shift the transmission as with the trucks I've owned before.
And in regard to programmers there are two other, less severe, but annoying issues:
1. The driver seat belt must be connected to put the truck in gear (Even if your just moving it around the driveway to hook up a trailer).
2. There are no "Hot" outlets (USB, plug, etc) when the vehicle is turned off so you can't run any accessories unless the vehicle is running. I have a 12 v fridge I use while camping, now I have to put ice in it.