How to check the catalytic converter with the oxygen sensors
Before replacing the catalyst, look at two graphs: often it is not the guilty part.
Get the car fully warm
A cold catalyst does not work at all — that is how it is built, not a fault. Testing only makes sense on a warm engine, after a few kilometres of driving.
Open both oxygen sensors side by side
You need the front sensor (before the catalyst) and the rear one (after). The front must swing roughly between 0.1 and 0.9 volts several times a second — that is how the unit trims the mixture. It is its job, not a fault.
Compare the behaviour, not the numbers
With a healthy catalyst the rear sensor stays flat, almost a straight line. If the rear starts copying the front's swings, the catalyst has stopped doing its job. That is what code P0420 is about, and it is visible by eye before any code appears.
Check whether someone earlier in the chain is to blame
A catalyst rarely dies on its own. Misfires and a rich mixture kill it: unburnt fuel burns inside and melts the honeycomb. If misfires or large fuel trims sit next to P0420, it is too early to replace the catalyst — a new one will die the same way.
Tell the catalyst apart from the sensor
A rear sensor frozen near 0.45 volts is usually the sensor itself, not the catalyst: the unit has stopped listening to it. The difference is obvious: a dead sensor sits still, a tired catalyst copies the front sensor's swings.