2006 Mitsubishi Lancer Oz rally P0421 fix

Question

Should I try changing the O2 sensor first? If so which one is bank one. Or is it more likely to be my catalytic converter?

Answer 1

P0421 Warm up catalyst belwo threshold

This is more likely will be the catalytic, You can make sure just by using a scan tool that can read data from the comfort of the driver seat.

 Using a scan tool with data, reviewing the rear 02 sensor a good catalytic will stay above 0.650 volts, yes it does fall below 0.650 volts but not too much

A bad catalytic that goes below its threshold will consitently fall below 0.650 volts.