if we 'assume' (dangerous I know, but a starting place....) EITHER; 1. the cat is not 'lighting off', or, the sensor is unable to report correctly (bad, cut wiring, etc.);
I have never needed to to try it, but if the cat is NOT working, the temp at its output will be the same or lower than the inlet side. If it IS working, the temp at the outlet will be HIGHER (by maybe 100*F) than the inlet. Try some measurements at the pipes with an infrared therm.
Or, get live data and look at the sensor's voltage.
One sometimes overlooked cause of P0420 is exhaust leaks.