Thread: cat or o2 on po420 on l26

Drove my car today on the freeway and notice my check engine light came up took it to the nearest AutoZone to get my codes run and the told me I had a po420 but confuse on which could it be the cat or the o2 how can I test my cat if is good I don't feel loss power but maybe I could be wrong my gp has 147k miles on it

420 is the rear O2 vs front O2 not showing enough of a difference. Therefore it would indicate a bad cat.