66GasHD

They messaged me earlier and said they are opening a case with GM engineering.

2024 GMC Sierra 2500 HD, 6.6L L8T gas, Allison 10-speed. Currently about 30,000 miles. Truck has had an easy life. I’ve towed maybe a couple hundred miles total since new. No heavy towing yet. On the highway it suddenly bucked hard, then immediately downshifted and stayed in 2nd. Felt like I slammed on the brakes with traffic behind me. I had to limp it at about 45 mph at 4500 rpm to the nearest exit. Transmission temp climbed quickly to 190°F before I shut it off, which is the hottest I’ve ever seen it (still not that hot). This was the first time the check engine light came on. After restarting it, I drove it about an hour to the dealer afterward taking only backroads. Dealer says they don’t know what’s wrong and that all the codes are historical. One of the techs looked at it while I was grabbing my stuff and said “this ain’t good.” They’re doing a test drive tonight and said they’ll consult their technical specialist. Has anyone else had this on a low-mileage L8T / 10L1000? Did GM actually fix it with the valve body TSB? Considering it was driving absolutely fine when this happened, I really don't trust the truck and its looking like the dealer isn't going to do anything about it.