how to check 02 sensors on 2002 Toyota Camry

replaced lower 02 sensor and cleared codes and after 50 mles ck engine light returned. I've been told its a catalytic converter but want to eliminate smaller possibilities before confronting the cat issue

Asked by for the 2002 Toyota Camry
the 420 code is cat effeiency. rarely is the o2 bad. not much else to check.replace with california approved or light will return. most aftermarkets do not meet oem specs and will turn the light on.

Thanks Roy I'll be sure to verify the exact code. Can you tell me whats the engine code for a bad purge control valve?