Sorry to read about these continual baby diesel problems. I loved my 2020 AT4 3.0, but when it was unfixed after a month in the dealership, stranding me for an extra month (was already gone for 4 weeks already as I was working out of state) with no fix in sight, I had to trade it.
GM needs to figure this crap out. I considered a 2023 AT4 HD diesel, but was reading about the similar issues with those also. Could have bought a 23 AT4 HD for almost the same price (probably a bit more expensive) than my 2023 AT4X, but I think I am done with GM diesels. Would love the have that 6.7 ltr AT4 diesel, its torque, power and yes, improved fuel mileage over my 6.2 AT4X (I do love the 6.2 but it is thirsty), but just don't need the headache of more possible issues with a more complicated fuel/emissions system that these diesels require. Hope those here with AT4 diesels are able to get GM to figure this crap out. It seems to happen to frequently, and often the same or similar issues people are having.