I went back to the dealer yesterday. Explained what I had learned form the forum. Their main technician thinks it is actually something to do with the DEF pump, and says he has seen this once before. Unfortunately, the guy is so busy that they can't even get to it for a week or two. Any thoughts?
The dealer is now telling me that there is a leak in the turbo, and the turbo needs replacing. So now they are in search of the part and will let me know when they get it fixed. GM has said that the turbo is covered under the power terrain warranty. So it won't cost me this time. I've been without my car a total of almost two months now. I hope this fixes the problem. I will keep you posted. I have 61K miles on my 2018 Terrain Diesel.