This story involves a complaint of no third gear on a 4T80-E transaxle. However, the lesson to be learned can be applied to almost all root-cause diagnosis and troubleshooting tasks.
The material for this article involves a unit that was built for our stock inventory and, at the dyno, had no third gear. This would normally lead me to believe that it had a stuck 2-3 shift valve or bad 2-3 shift solenoid. Simply pull the lower pan, swap out the solenoid and the problem should be fixed – or that was my first thought.