Originally Posted By: partspro
Not exactly 14.7 to 1. The ECU fluctuates the mixture between rich and lean which results in a cleaner exhaust. If you watch the O2 sensor you will see it 'toggle' or switch, many times a second.
Does that for one or two reasons (not completely sure if both are "required"): first, the upstream O2 sensor (the one before the cat) will see that wide swing. But the downstream (after cat) should stay at 14.7:1 if the cat is working properly. Once it starts to swing also, that tells the ECU that the cat efficiency is not as good as it should be. Second reason it may cycle like this is to run rich momentarily and make the cat do more work--by doing so will boost the cat temperature. I doubt this is needed at cruise speed on the highway but at idle it may be necessary to keep the cat hot.