Originally Posted by roadkillrob
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.