Check engine light

What is your question?

2015 Chevrolet Cruze ltz. My check engine light has been on for months. My car runs perfect and has no issues. I hooked the scanner up to it and it pops back P0420 and one other code. It says Catalytic Converter as well as o2 sensor. I reset it and drove a few days and it came back. I’m not sure which it could be and don’t want to randomly replace stuff without knowing for certain