Q: po420 issue!! on 2001 Infiniti I30

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I recently had o2 sensors code and got that taken care off i put 3 new o2 sensors in then i got the dreaded po420 code i replaced the middle catalytic converter which i heard was the right one then 2 days check engine light came on po420.. my mechanic said the pcm might need to be updated but i dont know what to do i heard that costs a lot of money .. any ideas????? also i beleive they put a universal cat in. if that helps
(1) Answer
This one is challenging; I can't tell you if the correct cat has replaced... And things are more complex since a "universal" cat may not solve your issue. So, even if the correct one was replaced, it may not be quality enough to solve our issue.

And, if you go back to the oxygen sensors - I would make sure those are the correct ones and installed well. Universal oxygen sensors, for example, may not be as good as original equipment components. It would be a shame if the catalyst has been measured as 'inefficient' due to weak oxygen sensor performance!

I think you need to find a trusted technician and shop to really unravel this whole deal and get to the bottom of your problem. Good luck.