A DTC P0420 is usually throw when a catalytic converter is blown, a plugged Cat
would cause Misfires, over heating, and could choke an engine out!
1) Have a muffler shop check your catalytic converter with a back pressure test, it's alot cheaper.
Because of Age + Mileage suspect bad plastic, rubber seals, and gaskets.
A smoke test can pin point vacuum leaks on your Intake, EGR, and PCV systems.