Question on likelihood of Cat converter vs O2 sensor repair

First try some catalyst cleaner, you can get it at most parts stores. Usually when that code shows up the catalytic converter needs replaced but I have used this stuff and it can work if the catalyst is not too far gone.

Code P0420 tells you O2 sensor is doing it's job.

It's the catalytic converter. P0420 is telling us that when the PCM (Powertrain Control Module) tests the catalytic converter, the test fails. The oxygen sensor is working. Hope that helps! Jim