P0420 code.What could be the root cause for the cat to be going bad?

Question

Runs fine, mileage gone down, what else might be the root cause?

Answer 1

P0420 Catalytic below threshold Bank 1

Some of the causes for catalytic to degrade sooner than expected are from misfire, coolant seepage into the combustion chamber  , and poor engine maintance. Then there root causes this is the push of the oil change industry for the use of synthetic oil when the engine does not call for it.