I have a 2003 passat 2.8 I have a car with a p0431 the guy said the pid for downstream sensor was fluctiating and that it should be steady so he reversed the harness conectors between bank 2 and 1 and the pid was fluctuating in bank 1 so he knows its not the catalitic converter we had already changed both downstream sensors so we know its not that the only thing he found was that a pin for the bank 2 downstream sensor harness was broken he went to the junk yard and cut a pigtail harness to add to the old broken one did he do something wrong connecting the splicing the harness we know its not the catalitic converter or downstream o2 sensors
Your mechanic did all the right things. Not sure why you are asking a question.
Did the problem get fixed? If it did, drive on. If it didn’t, the damage at the pin is now fixed and your mechanic can move to the next problem.