Code P0431

I have the car listed above with a 2.8 liter engine. Code p0431 the guy said the pid for downstream sensor was fluctuating and that it should be steady. So he reversed the harness connectors between bank 2 and 1 and the pid was fluctuating in bank 1. So he knows it is not the catalytic converter. We had already changed both downstream sensors so we know it is 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 it is not the catalytic converter or downstream o2 sensors.