Trouble code po420

The P0420 code means the converter isn't working as well as it should, not that it isn't working at all. Usually the converter has to be way past the point of a code before it will generate that smell. That said it's possible the O2 sensor is reading a bit off but normally that would show a code before the P0420 would happen. Testing it would be easy with a scan tool able to read live data as you could look at the front and rear O2 sensors and see if they are tracking together and what the voltage levels are. Lacking that tool you could replace the sensor and see what happens. I'm going to say that it's more likely a bad converter, but testing to be sure of that would be the first step. A somewhat quick check would involve using a non contact thermometer to see what it is doing.
https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

It is normally a converter but I have seen upstream/downstream o2 sensors misread the exhaust and cause the PCM to think the converter is less efficient. I attached the testing but this is not conclusive to rule out the o2 sensor. Normally I would tell you to swap them to the other bank but you only have one so that won't work. Based on that info, I would just replace the o2 sensors and go from there.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Let us know what happens.

How do I know if it’s the catalytic converter or the O2 sensor?

The only way to know for sure is to do one of two things, test the parts to find the failure or replace one and hope you guessed correctly. If you don't have the test equipment and think a shop is too expensive you could swap the rear O2 sensor, clear the codes and see if they come back. Not the method I use but I also have the tools to do the testing.