Hold on. If they failed to fix it while it was in warranty, then they still owe you a fix, if the code is the same one as they "fixed". A trip to Small Claims Court might settle this argument.
But aside from all that, what you need is someone *competent* to diagnose this problem. There is a diagnostic tree of step by step instructions on how to debug this code, all the way from testing the 02 voltages, to checking for loose, corroded or shorted wiring to replacing the PCM module.
Code UO404 is addressed by a NAG 1 Transmission Verification Test
These are not necessarily easy problems to track down, so you'd best insist on your warranty. You can't bail out of a broken car, you'll get creamed on the resale. It's time to dig in your heels and demand satisfaction or go to Small Claims if they refuse--my opinion is that they didn't fix the problem during warranty.