Theory Of Operation
WARNING: This page is about a different car, the 2012 RAM Pickup. However, it is still accessible from the selected car via links, so may be relevant.
For an aged O2 sensor, the response rate to the air/fuel change is slower than when it was new. The O2 sensor tends to move less with the same air/fuel changes in a given time frame. Therefore by observing the activity of voltage readings from the upstream O2 sensor, the quality of the O2 sensor can be detected.