I have the same issue also, 2016 denali Duramax idle at 950 rpm most the of time and does Not throw any codes either. finally bite the bullet brought it in dealer hopefully the computer at the dealership figure out the issue. Unfortunately could not do shit, so after 2 days sitting at the dealer they finally found the engine harness was rubbing against the egr/ac condenser causing a shortage. they replace 2-3 wire were burned down to copper. problem solved and cost me $525 plus tax. all they did was using electrical tape and zip ties the harness out of the way.