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Originally Posted by 23109VC
i dont' know what would happen if you took of the fronts, but left hte secondaries.
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The biggest problem you would now face is that you have a set of O2 sensors BEFORE the primary cats (the ones integrated in the headers) and another set BEHIND the primary cats.
So here's what happens:
The after markets headers have usually only one set of holes for the O2 sensors so your second set would dangle in the air. To make things worse the DME will be very confused as both sets of O2 sensors now read the same values (and the DME usually expects the air before it goes through the cats to be dirtier than after). So if you just leave t like this you will have a check engine light and probably poor performance. But there are ways around this, do some search on the forums.
But long story short: You do not run into these issues if you leave the original headers on and replace the secondary cats with a bypass pipe, and this is also mechanically less challenging (there's a bunch of guys who had serious trouble with the studs with which the headers are attached to the motor)