emission help
 

i have a 2001 audi a6 quatro 2.7 car with 130 thousand miles I am in emisson hell this year for inspection. I have replaced all 4 ozygen sensors now with the last being a day ago. The check engine light came back on again after 20 miles. This time i got a p0431 code that I was told would have to be a leak in the flex pipe since the cat appears to be working. i have not had the audi dealer check the cat, but my trusted mechanic said it was working properly.

What are the chances that I do have a bad cat ??? I am getting the bank 2 flex pippe fixed on monday, which is cheaper than the cat, but 4 ozygen sensors later I am tired and exhausted from the hit and miss. The four sensors were over three visits with the last being at the audi dealler that had the banks of the car mixed up when they ordered the parts originally

any help is appreciiated