2002 Honda Civic EX check engine light. (P0420)?
 

I've a 02 honda civic ex & the check engine light is on. It came on after I put a full exhaust system with headers on. I changed all the O2/ oxygen sensors on it and it still is on. The DTC code that I got was P0420 Catalyst System Efficiency Below Threshold. The exhaust system is a racing type. I dont think its reading the excess airflow into the engine correctly. Any suggestions? Thanks in advance.

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