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

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?

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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