P0420 sounds like a verdict: catalyst system efficiency below threshold. It is easy to read that as “replace the catalytic converter,” especially when a customer is waiting for an answer.
But the code reports the result of a monitor. It does not, by itself, explain why the monitor failed.
Honda made this distinction in a service bulletin for 2014–2016 CR-Z models. For a P0420, Honda instructed technicians to update the engine-control software, test-drive the car and check whether the converter actually needed replacement. The bulletin said a new converter would be unlikely after those checks.
That does not mean every P0420 has a software cause. It means the name of a fault code is a place to begin, not a parts order.
The scan tool has done useful work: it has narrowed the search. The technician still has to establish the cause, choose a repair and check that it solved the original problem.