The whole point to the non-fouler thing is this..
The reason you get a CEL with a header is because there is no cat. restricting the flow anymore.. the O2 sensor sees all the extra emissions going past and throws a CEL..
when you install the non-fouler, wait.. im getting ahead
when you look at the non-fouler, you notice it only has a tiny hole..
you drill out ONE side only to make room for the o2 sensor to fit.. and leave one so that only a little bit of exhaust gas can get in.. the normal amount the o2 sensor would see if there were a cat.