If you use a O2 Cheater it will not. Cost like $35.00 and can get it from just about any of the Focus parts dealers. I just had a FC race Header installed on my car and used the O2 cheater, and no problems what so ever. One more thing, you are going to love your race header, it will make a different.
you have to use the O2 cheater for the use of a race header.because the o2 sensors are right beside each other on the header since the cat is no longer there to read clean and dirty.this allows to bypass what the o2 is reading and not throw a code.
Either way you are fine...with or without the o2 cheater. I ran the FC Race Header with no o2 cheater for a month(4K miles) with a CEL and no problems. Then put the cheater in and it was the same just no light. Now again I am running without it with a light. The race header freakin rocks...you will have nothing but grins the first week!