"P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
Indicates Bank 1 catalyst system efficiency is below the acceptable threshold
Use of leaded fuel
Malfunctioning ECT sensor
High fuel pressure
Damaged exhaust manifold
Damaged catalytic converter
Downstream HO2S wires improperly connected
Damaged exhaust system pipe
Damaged muffler/tailpipe assembly
Retarded spark timing
Compare HO2S upstream and downstream switch rate and amplitude. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the downstream HO2S very slow and reduces the amplitude of those switches as compared to the upstream HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines and the downstream HO2S signal begins to switch more rapidly with increase amplitude, approaching the switching rate and amplitude of the upstream HO2S. Once beyond an acceptable limit the DTC is set."
imo, there are really only two causes for the code if the car is running well and there are no obvious problems as suggested above; the catalytic converter is well and truly done or the rear O2 sensor itself is failing (or there is a rear O2 sensor wiring fault. The rear O2 is relatively exposed to road/weather.) With your accumulated mielage, either possibility is valid.
O2 sensors are relatively cheap. Catalytic converters are not.