It looks like the catalytic converter is bad, this is why you get this code, the oxygen sensors will not change this. It is probably why the car is so loud as well.
Code P0420 on a 2008 Nissan Altima indicates a Catalyst System Efficiency Below Threshold (Bank 1).
I would remove the exhaust manifold for inspection, here is a guide that can help:
https://www.2carpros.com/articles/bad-catalytic-converter-symptoms
Here is what I am talking about. I would look for loose catalyst material inside the exhaust system. Check out the images (below). Please upload pictures or videos in your response of any problems so we can see what to help you with.
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Tuesday, January 28th, 2025 AT 10:15 AM