I have 2300 miles on my 2021 RST Duramax, have same situation. Dealer said they could not test the trucks electrical system , because I had an after market sound system installed. I would need to remove the system to properly test, I then told them the no start issue happened before I had the system upgrade. Dealer kept the truck another day to have another technician look at it. I Love truck and have big plans for it ( lift tires wheels). all that has to wait. I just wish the dealer would come out and say they have problems rather then try to blame me for making upgrades!