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Here is a quick explanation of what O2 sensors do:

O2 sensors are basicly a 14.7:1 air fuel ratio switch. Imagine the O2 sensor turning on a light bulb every time the air fuel ratio is 14.7:1 (or close to it).

Now durring closed loop operation (not at WOT or start up) the EEC "reads" the O2 sensor anywhere from one to ten times a second. (or maybe its 20, I have to look it up). Anyways, here is what happens when you're going down the road.... the EEC leans out the mixure, and when it gets close to 14.7:1 the "light bulb" so to speak goes off from the O2 sensor which tells the EEC its at its ideal fuel ratio, and to rich it up a bit... so now the EEC riches it up a bit since it has now shot a bit past 14.7:1 and gotten into the lean 15:1-ish range. When the air fuel ratio gets to 14.7:1, the "light bulb" so to speak from the O2 sensor goes off again and tells the EEC, you've gone past 14.7:1 and gotten into the 14:1-ish rich zone now, lean it up! The cycle repeats itself one to ten times a second! As you can see, this keeps the air fuel ratio 'hovering' at 14.7:1 while cruising, keeping the tree's alive and your gas milage up! Pretty cool eh?

Now when you floor the gas, the air fuel ratio drops to the 12:1 to 13:1 range and the EEC switches to open-loop mode. O2 sensors don't read air fuel ratios this low, so the EEC ignores the O2 sensor and calculates the air fuel ratio from inputs on the MAF and built in 'tables' (for the most part).

I know I've skiped over a lot of technical details here, but I'm just trying to give everyone an simple, yet effective idea of how O2's work.

-Sawson

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