I have a 1999 Chevy blazer and it runs good it has 224,000 miles on original motor and trans. But the lady I bought it from over filled the trans a lot. Like a ton. So with my oil dipstick pump I pumped out some fluid. But it still has this problem where when it gets hot after about 40 mins of driving it dosent down shift from 3rd. I can manually shift it into first and then to drive and it’s fine. I have a U1193, U1192, U1042, U1026 codes I want to see if it’s something simple before I pull the trans and rebuild it

 

Welcome to the forum,
With the U codes you have listed I would lean towards that the PCM is losing power at some point. It takes a millisecond for the PCM to set these codes when signal is lost. Do the lights for the shift indicator in the dash work?
Brad