"The O2 sensor is constantly in a state of transition between high
and low voltage. Manfucturers call this crossing of the 0.45
volt mark O2 cross counts. The higher the number of O2 cross
counts, the better the sensor and other parts of the computer
control system are working."
http://www.birrabrothers.com/tiger/e36/docs/all_o2sensor.htm
Joe