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Q:

how can you tell if the rear catalytic converter needs replacing? on 2001 Nissan Sentra

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mechanic used computer to scan check engine light problem - said I needed rear catalytic con, which cost me $400 for part and isntallation. Truth is, I needed a new engine as I had cracked head or blown head gasket - due to bad gas. I did not need rear cat con, why did code read that way?
(1) Answer
Your defective head gasket poured out way too much pollution and over worked the CAT. This is common. You most likely had a P0420 code. Check out this article I wrote about this code, it may help shed some light.

http://repairpal.com/OBD-II-Code-P0420