Engine start-up I kept getting the code P0420?

Tiny
KBIRDWELL97
  • MEMBER
  • 2008 NISSAN ALTIMA
  • 2.5L
  • 4 CYL
  • 48,699 MILES
I have a 2008 Nissan Altima I kept getting the code p0420 which resulted in me getting another transmission (used) got that put in I can tell the difference got new coils and spark plugs put in around the month of November also got a new catalytic converter put in front and back also. Now I keeping getting the code saying it's my downstream o2 sensor I bought 2 new 02 sensors and had them put in also got a code for crankshaft sensor and I bought that new also and had that put in I keeping getting the code for the 02 sensor even after replacing it. Every time I start my car up it's extremely loud and you can hear my car running a mile away where do u think that noise is from I thought it was coming from the engine but it's not sound like its coming from the back. Every time I start my car up its constantly cranks and cranks and I can hear it try and catch and try and turnover and most of the time it does and sometimes it will sound like a coughing effect and then turns off what do you think the problem is and why I am I still getting the code for m02 sensor after I already replaced both?
Monday, January 27th, 2025 AT 6:31 PM

1 Reply

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 49,127 POSTS
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

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