Here are a couple of interesting reads for those of us who were not sure how they worked.
http://www.autotap.com/techlibrary/understanding_oxygen_sensors.asp
http://www.aa1car.com/library/wraf.htm
http://www.autotap.com/techlibrary/understanding_oxygen_sensors.asp
http://www.aa1car.com/library/wraf.htm