I bought a 2000 Chevy 1500 from my uncle's estate. It only had 84,000 miles on it and had been sitting up several years. I drove 200 miles from Atlanta, GA to my home and it operated flawlessly. Once I got it home i started changing all fluids, both differentials, transfer case and auto transmission. As folks may know, removing the transmission pan is not easy with the shift linkage bracket in the way. I was able to remove the transfer case shifter from inside the cab to access the Torx bolt and move the bracket to get the pan off. I thought everything was put back together properly and when I went to test drive it, before I could get out of the driveway a P0756 code popped up. I made sure the fluid was to the right level but the transmission will not shift out of 2nd gear. It worked fine before I "serviced" the transmission. Any ideas?

Thank you

 

try the troubleshooting steps here
https://www.obd-codes.com/p0756