allhalf425

I recently had my car looked at and was told I needed to replace my front-right cat and one of the o2 sensors. I already got the front-right cat, but I don't know which o2 sensor I need to have replace. I forgot to ask them, and they don't have anything specific on my paperwork. I really don't want to have to pay for another hour of labor to find out. Any ideas which o2 sensor it would most likely be?

It was throwing P0420 and P0440 codes, if that helps at all. Thanks.

Edit: Might help if you knew what car I was talking about...'01 i30