PO420 code

[suddenplumbing]

hello,

i'm new to the forum and learning alot. i have a 2001 cavalier with
a code of po420. sounds like its the cat or the oxy. sensors. are there any other choices? the car has 53K on it and the dealer replaced the cat once. seemed pretty strange since the car only had about 30K. then. car seems to run ok but the CEL has been on for two weeks.
thanks in advance for your help,

[Joe W]

This code is usually an O2 sensor,but considering the history it should go to the dealer.(should be under warrenty.