Just to add a link, this page:
http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm has, about 1/2 way down the page, a very nice description of the P0420 code, why/how it generates and an explanation of possible causes. Makes clear that the code generates from a comparison of the rate at which O2 is converted by the cat, if I read this correctly, from comparison of rate of change of O2 content pre and post Cat. Frankly interesting in and of itself.
RAS