Got a brand new 2022 Yukon XL with 3.0. Thought I smelled diesel fumes at the dealer but brushed it off, because hey I'm a farmer. Wife cried when I brought it home and said she couldn't handle the smell. Needless to say it became even more apparent there was an exhaust leak in the engine compartment which would end up also stinking up the cab. Read this entire forum and it was not the exhaust check valve or the clamp on the bottom that have been listed on this thread. I located the leak at a clamp directly in front of the turbo. It was literally puffing out exhaust and there was soot by the clamp. Took it to the dealer who put on the new clamp and new gasket and they said it didn't work and we don't know what to do to fix it and they would have to call GM. I then decided to look at my friend's 2020 Sierra with a 3.0 which revealed the problem. The clamp has a metal strap that holds the two halves together. When you tighten it up it cinches the clamp tight which pulled the two pipes together. My factory clamp was welded together with a very large gap(about half an inch) between the two pieces while my friend's clamp was welded with the two pieces almost touching. This made it almost impossible for my clamp to effectively create a good seal between the front of the turbo and the exhaust pipe it is clamped to. I got desperate when no one could or would help me so I took it home, removed heat shield, took the clamp off, drilled out the two spot welds on one side of the clamp and rewelded that same side with the two halves of the clamp almost touching. I then loosened the brackets on the part of the exhaust pipe that went farther down into the engine compartment so that the clamp would be able to effectively cinch up without being restricted by the brackets. I then reinstalled the clamp and leak tested it with soapy water. Voila! No leak. I then retightened the brackets and put it all together and leak tested with soapy water again. Still good! The shocking part was even the replacement clamp from GM was manufactured incorrectly with too big of a gap. Which is why the GM service dept couldn't get it fixed. I'm sure they didn't try too hard anyway. I will include pictures as it was with a large gap when it leaked exhaust and i will also include pic of my fix. It's not perfect and doesn't look beautiful but at least it doesn't stink anymore. Shouldn't have to do this with brand new vehicles but that's just America today..... Best of luck to all.