The code is thrown by the O2 sensor downstream from the catalytic converter returning a voltage reading that is outside of the ECU's acceptable range. The O2 sensor checks that the cat is still doing its job and it doesn't affect anything noteworthy.
In order of probability, it could be:
1. The O2 sensor wiring. It gets snagged on things off-road and it's exposed to the elements. A visual inspection usually is enough to check if it is the problem.
2. The O2 sensor. They are pretty long-lasting and simple but I have had one fail. Unless you have access to a way to read the O2 sensor voltage from the ECU, swopping it out is the only way to confirm it was the problem.
3. The catalytic converter. The metals inside it melt and solidify if the engine has been run very hot. If you bang on the cat, you hear the chunks of metal rattling around in it.
4. The ECU or ECU program. It's so unlikely as to not even be mentioned again.
Remember to reset the OBD2 error codes when you are done with your repairs. P0420 doesn't go away quickly (when the ECU sees all good for long enough).
The average human walks 1,500km per year and drinks 83L of beer which means we're getting 18km/L.