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Discussion Starter #1 (Edited)
So I recently purchased the mid length header and down pipe with cat from zzp and a day later I got CEL. Pulled the code and it was a P0420
(Catalyst below efficiency) ? Or so I read. Does that mean the cat on my new down pipe is bad?
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Not necessarily, I've read since it's a high flow cat your downstream o2 sensor might not read it right and throw you a code, I get it too but I just clear it, it comes back after like 3 weeks yes but that's fine until I ultimately get that code removed with a tune
 

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From service manual:

Circuit/System Description

Important: A new converter with less than 100 miles on it may set DTC P0420 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.

A 3-way catalytic converter (TWC) controls emissions by converting hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide (CO2), and oxides of nitrogen (NOx) into nitrogen. The TWC also stores oxygen. When the listed driving conditions are met, the Catalyst System Efficiency test will run. The powertrain control module (PCM) monitors this oxygen storage capacity by comparing the pre and post-catalyst oxygen sensor signals while adding and subtracting fuel during specific driving conditions. The PCM compares the pre and post-catalyst oxygen sensor signals to determine if the oxygen storage capacity of the catalyst is degraded.
 
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