This video is for you to diagnose the problem with your 4L60 transmission when 3rd or 4th gear won't engage, we show the parts that typically are bad when you can't get 3rd or 4th to work in your transmission.

4L60
4L60E
Won't shift to 3rd or 4th
Slipping in 3rd or 4th gear
Disassembling a 4L60 transmission