Did your fans work properly before you installed the FanZilla? And if so, did converting back to the old stuff get it working right again? Keep in mind that your otterstat may not be sensitive enough. Mine doesn't click on until the coolant temperature reaches around 220 or sometimes a needle width higher. I have a new one to put in but haven't gotten around to it, and I almost always run the a/c here in Florida. To rule out a problem with the otterstat, just switch the a/c on. But remember that if your a/c doesn't work (i.e. the compressor doesn't kick on due to low refrigerant) then neither will it trigger the radiator fans to come on. Another thing to look out for are the female spade terminals in the relay sockets backing out and falling down. I've had that happen to me before. Walt Tampa, FL