what happens with aftermarket headers is that it eliminates the cat, thus having more exhaust pressure on the 2nd 02 sensor the check engine light comes on. With the anti fouler your trying to take the sensor a little farther back(outwards) so it doesnt read that much exhaust pressure ,therefore reading the same exhaust pressure as if it had a stock manifold
*sometimes it needs 2 antifoulers together*
*this was the cheapest way out but it has worked for many of my friends*

The other remedy that you have is to buy an 02 sim card or cel eliminator, this is more expensive but it also works.I suggest that you buy it from the company that sold you the product, if the company doesnt have one try an anti fouler . If you wish to buy a 02 sim card or 02 eliminator , be sure that wich ever it is works with the brand of header you have,I've seen alot of cases that these things dont work on some kind of headers. HOPE THIS HELPS OUT!, GOOD LUCK!