M31991

Hey everyone,



I have a 2014 Mazda 3 2.0L isv model



I have done the TSB ECM update for this code (p0421 bank 1)

i have changed both o2 sensors

everything but attempt to replace the catalytic converter for this code



to my understanding this car has 2 catalytic converters (1 by the exhaust manifold and 1 under the car by the tail pipe)



how am I supposed to know which one to change?



the code is p0421 bank 1



thanks!