What is required to do and what is the approximate cost to correct a P0420 Mazda problem

Question

Vehicle is a 1999 Mazda Protege. 1.8 litre, 5 speed, 4 door.

Answer 1

if there are no exhaust leaks, the most common problem is a bad catalytic converter, which might be $250 for an aftermarket catalytic converter or more than double for a OEM one. But keep in mind the OEM catalytic converter will last as long as it did and aftermarket a few years in many cases