I swapped out my old 5.3 for a new one in my 2005 GMC Z71 CC and in the process, I pulled my 4L60e and did an overhaul on it. I only replaced clutch packs, seals, put in a corvette servo, 2-4 band, and cleaned it up. The only issue in the entire process is my truck is throwing a P0785 code, and doing this:
- stays in 3rd gear
- will not shift up to 4th
- will not shift down to 2 or 1 automatically
- will shift down to 2nd manually, but not 1st
- if the DTCs are cleared, I can start the truck and it will shift 1, 2, 3 just fine, but then stay in 3rd until the codes are cleared again

I'm not sure what this code is exactly, everything I've searched said something slightly different. The general census is that it's an issue with the circuit. I'm going to use connector cleaner on the wiring connections and cross my fingers that it fixes it. Anyone have this issue before?