well theirs your problem. as far as I know, auto parts stores dont sell O2 monitors, they will try and sell you O2 sensors in place of O2 monitors and they are NOT the same. second, and I preach this on a daily bases, you CANNOT buy Sensitive Electronic Sensors for Fords at parts stores, you have to go oem. FoMoCo makes it sensors to work wthin certian parameters (Y----Z) and the aftermarket opens up these parameters to make there part fit multy vehicles. there parameters might be between S------------Z. its just not the same.
randy knows what he speaks. on a side not, you wouldnt happen to have non OEM cats on your car? something like a catted X or H pipe, or maybe a off road pipe? if you do thats your problem as well.