behind a cover on the driver's side of the dashboard

What is your question?

Hello, I have a 2010 Nissan Murano showing a p0420 code. It says catalyst system efficiency below threshold (bank1). It was running fine, just noticed some decrease in gas mileage. The other day it began to feel as though it had a mild misfire or sputtered. It has me concerned, and I am not sure what to do. I am laid off so if I can bypass a serious replacement I would prefer to do so. Would it be my catalytic converter on bank 1 for sure? Maybe the o2 sensor? Or the 3rd cat where both pipes run through together? How can I know?

What makes this problem better or worse?

Once the vehicle has warmed up enough to operate

How long has your 2010 Nissan Murano had this problem?

A week