Aldive is right. In theory the sensor will, at high temps (which occur after a few minutes of running) produce voltage. Your computer relies on that voltage to adjust the fuel mixture. The "cross over" voltage is about .445 volts as I recall. It will bounce back and forth above and below that voltage as the computer continually adjusts the fuel trim in the "closed loop" mode. The upstream sensor should crossover at least 8 timeas a minute... downstream sensors are far more stable. An OBDII scanner can actually show you the readout for sensor voltages, so you can see if your sensor is getting lazy or what. Frankly I do not know why more people don;t take the money they waste replacing perfectly good 02 sensors and BUY a damned good scanner using that money and throwing in a few extra bucks. But you can have someone else do it too. Using the above, see if you cannot get a look at the voltages it's putting out while the car is running.
Oh for those who might nit pick, those voltages are for Zirconia 02 sensors, a few imports and Jeep as I recall used Titania sensors and their crossover is more like .25V.
Hope this made sense and helps.
Happy exploring
Chris
