[Tgo213]

I have an 03 audi a6 and it driving me crazy. I keep failing inspection and I'm not understanding. The first time I failed the codes read that my front two o2senseors was bad so I changed them. the check engine light went off and i went back through. I failed again and now it's saying the passanger side back o2 senseors wasn't working fixed that. It took about two to three days to come back on, but it's on now. I haven't checked the codes yet, but I noticed while the light was off for the two days it was still codes that came up saying the heated catylist on the same side that I changed both senseors. Can anyone explan any of this? I'll try to get the codes that it's reading now. thanks