HELP! 03 impala won't shift

Keep getting code P0753 on an '03 impala that we've changed the transmission, solenoids. Started running ok but after got on highway it wouldn't shift to fourth. Got home, turned off and when restarted wouldn't shift at all. What else can I check? Would flashing the computer help at all? Getting very frustrated don't want to have to take in and pay even more.

A full trans scan will look at what is going on in the unit, anything else is guesswork.