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Message: <blockquote data-quote="GaryM" data-source="post: 31207" data-attributes="member: 9373"><p>Hmmmm. I have a friend who worked at GM test track. His take on this whole thing was the original download to the TCM was corrupted by a disconnect before the download completed. Given the nature of the defect it became pretty obvious, at least to him, the issue wasn't mechanical. The fact that 8th gear can be achieved and held w/o issue points to a programming error or exclusion in the parameters coding for 8th gear acceptance. We're going to reprogram the TCM Wednesday and I'll make sure this update you reported is included. We also found the transmission reacts to ATF when it's colder than 150 deg F. Previous models (6 spd) used a dual role ATF cooler to heat it with coolant during warm up. Starting in 2025 they went to a cool only system allowing the transmission fluid to warm on it's own. That's a slow process especially when it's near zero like it is around here. Mine was sticking in 6th gear, ignoring 7th and 8th all together. I changed the ATF the factory uses to a product Mobil 1 makes. It's still a GM approved Dexron VI, just better cold weather properties. That helped quite a bit. Still didn't get me 8th gear, but all the other shifting performance has improved. I still have to use a technique to get 8th gear. Once there it performs very well.</p></blockquote><p></p>