Oxygen Sensor

Hi,

I've been having problems with my oxygen sensor that I hope someone can give me some advise. I own a 1998 Civic Ex with AEM CAI, DC Sport 4-1 Ceramic Header and DC Sport Catback. Since the header was installed two years ago, I've gone through three O2 S1 Sensors. The engine code is PO133.

The dealership is telling me that the reason my O2 sensor is falling is because the position of the O2 sensor on the DC Sport header is 6 - 8" further back then the OEM exhaust manifold.

Has anyone ever heard of this problem, or is there another reason?

Mike