I'm not sure what it is but I would call the guys you bought it from at FC. They're usually really great at doing their best to help you out over the phone and they'll be able to tell you something. Let us know what's up...I've got a race header and O2 cheater sitting in the garage awaiting my motor swap.
Did you try it on the blue o2 sensor.. I'm sure on mine it is on the blue one not the green one. I know some people swap them around after installing because of the length of the wires, but the cheater goes on the one that was after the cat before installation. I have a homemade one on mine and no CEL.