I have an 04 duramax 2500 I just bought last weekend. It has 70000 miles on it, it has a ats 3 stage transmission and a few other things done to it. I am on my way home from camping pulling the camper and it starts acting funny. Its like it doesn't have all the power. So I stop shut it off start it back up and go to put it in gear and it wont go in and I get a code on the dash that says RANGE SHIFT INHIBITED. It finally goes in gear and I get going but it seems like it wont shift up to the next gear. I can only go about 40 mph and it is almost red lining. Im not driving it and waiting for a tow. Does anybody have any idea what it might be.

 

NSBU switch
(Neutral safety back up switch)

 

check for codes, if you get a P0780 code, you most likely need the NSBU switch

 

for sure NSBU switch