I got a CEL while on my way to work. P0420 Catalyst System Efficiency Below Threshold Bank 1. i recently just Put in a HFC and Borla Cat back system 3 days ago. Is it possible i damaged the O2 sensor while doing it? Or maybe the new system is still burning out everything in the cat still because my exhaust still has a funny smell to it like it did when i first put it on.

 

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I would say the HFC is just not doing its job well enough to avoid a cel. You might want to try a short spark plug non-fouler between the HFC and secondary o2 sensor

 

i did a little more research on the code and yeah its just not doing its job right but thats fine with me because that means better performance but i hate the damn light. Sorry to sound dumb but how do i or what is that your talking about "short spark plug non-fouler ?

 

I would have to agree with this

 

Here's a link to an article about the problem. I had the same issue when I first installed my APR 3" exhaust on my TT. I had APR flash a cat delete file to my ECU and that solved the problem, but here is a cheap fix that will work as well.

Spacer Fix

 

Thanks beefyosito that helps out a lot.