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      01-26-2013, 03:52 PM   #7
Stupid Idiot
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Drives: E90 335i & C6 Grand Sport
Join Date: May 2009
Location: Santa Barbara

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Originally Posted by roadkillrob View Post
If you have catted downpipes, why do you need O2 sims - I have AR catted with all the O2 sensors installed and it passes OBDII smog check no prob.

Do you really mean you have catless downpipes?

I've used high-flow cats on other cars that still cause catalyst efficiency codes. I don't have experience with them on the N54, but whether it sets a fault or not really depends on how well the cats clean up the exhaust. Aftermarket high-flow cats definitely don't work as well as OEM in that respect.