i've never gotten that code. 420 is catalyst effeciency. sounds like the same, and 99.9% its the cat. oxygen sensors tell the computer how the cat is working. when an 02 goes bad, the computer recognizes bad readings out of threshold, or abnornal, and throws code for that. the only way it is oxygen sensors is if two sensors are bad, and they are just barely bad giving the computer low readings.
if you have a scan tool that you can graph the 02's, watch them, if they change at a normal pace, but they are equal most of the time, the cat is bad.
cat's aren't cheap. they should be under warranty for 8 yrs/ 80k though.