I agree with Bucknut. I had my cat go bad last Thanksgiving and just for grins, the dealer quoted me $2000 for their part, which just so happened to be the last one in stock.
I went to my trusty muffler shop and for $350 within an hour or two I was cruising down the road with a high flow cat. It's probably not your sensors (unless you already know they're bad). Just get the cat and be done.