P0420, either the O2 sensor or a clogged cat converter. was told that the code is usually the O2 sensor

 

In my experience o2 sensor codes rarely mean you have a bad oxygen sensor. A bad oxygen sensor will set that code, but it will also set if your car is running rich or lean or misfiring. I'll give you an example. My freind was messing with a 4 runner the other day and it kept throwing oxygen sensor codes, and he replaced both of them. Code came back in the same usual time. Turned out to be dirty mass air flow sensor, which caused it to run really lean all the time and in turn made the oxygen sensors read lean all the time, instead of fluctuated rich lean rich lean when the ecm is in fuel control. Sometimes it's easier just to change the o2 sensors and hope for the best.