More than likely there's something wrong with the wiring to the sensor. Check continuity, and if you de-pinned a connector to go through the firewall or something, double check the pins are in the right order/cavity number. Also check that pin3 on the wideband connector (middle terminal furthest from the clip) is being supplied 12v.
Heater power and wiring problems are by far the most common reasons for wideband acting odd.