h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

"Footnote 3: Note that a worn-out oxygen sensor will swing slowly, or not at all, and that a new oxygen sensor should swing at least five times in ten seconds."

Actually, any oxygen sensor which has only five crosscounts in ten seconds (one in every two seconds) should be thrown away.

(Crosscounts, or "swinging," as it's referred to in the article, refers to the amount of time the analog oxygen sensor signal crosses the 450 millivolt threshold, thereby signaling a stoichiometric A/F ratio)

The amount of crosscounts usually increases with engine speed as a function of the oxygen sensor's increased efficiency, resulting from the increased heat applied to the oxygen sensor.

The amount of crosscounts will naturally always be lowest at idle, but even then, it should be AT LEAST one crosscount per second, even on early closed-loop systems. Under load, OBD II-equipped vehicles can have crosscounts of 40 per second and even higher.

The PCM can only correct A/F ratio as fast as the oxygen sensor can report it. Remember that the efficiency of the catalytic converter is limited by the response time of the oxygen sensor.