My dealer was in contact with GM engineering today. They see there is a problem on some trucks but have no idea what to do. Some trucks seem to respond to a different brand of Def but some do not. Changing Nox sensors seem to help some trucks for awhile. The air cooler pipe in a few were found at fault. This is sad for such an otherwise great truck. I'm hoping something develops soon. Truck has been in shop more than two weeks.