The best prices I found for our O2 sensors is urdusa.com . ~$85 for the Denso brand. I think I got the link off of this board at one time or another. But before I dropped that kind of cash for new sensors, how old are his spark plugs? A misfire will send raw fuel down the line to the cat. If you send more than the cat can take care of you'll end up with the P0420 code. My wife's 01 Odyssey threw that code off and on for a couple of weeks. I finally broke down and replaced the 95k miles original plugs (the plugs are platinum so they should last about that long) and not only does it run better (peppier) but she hasn't been greeted with the CE light since then.