My 10R80 transmission was flawless until 95,000miles. Then it began slamming into second gear. Harsh downshifts into 4th, 5th, 8th. Very violent engagement into 4th gear. Loud bang. Slipping while engating 5th. The transmission deteriated to a point where it was not drivable. Dealership determined it to have failed and required complete replacement. Cost, $3,000 replacement transmission, $3,000 labor. Total, $6,000! This is out of pocket as I didn't have extended warranty.