This may be due to an engine tune up issue and the O2 is just the symptom and not the cause. Do a general check-up on the engine before assuming the Cat is bad.

Answered by Zue
8 questions asked

What you have is an o2 sensor before and one after the converter
and when reading of those sensors is close to each other
that is indication that the converter not doing its job burning the access fuel left unburned
and you have to replace the converter

Answered by BMRFIXIT (expert)
19,736 answers provided