what can cause the codes 420 on both banks and 430 on 1 bank of a 2004 nissan murano

Question

It has new plugs and coil packs

Answer 1

Hi there,

The P0420 would only be for bank 1 and the P0430 is for bank 2 catalyst below efficiency.

Typically since the issue is on both converters, either the converters are failing, there is a rich fuel mixture, possibly caused by a dirty air filter or bad mass air flow sensor or there is a fuel additive that is causing the codes. I would start by clearing the codes and seeing if they return. Exhaust leaks can cause these as well.