The O2 sensors in a gasoline engine car works with the Engine Control Module to determine the oxygen content of the exhaust. This information is used during closed-loop operation to adjust the fuel injected to make the best charge for the conditions of operation. Except in modern OBD-2 (after 1996, I believe) there is a second O2 sensor behind each cat to monitor "catalyst efficiency".
Very helpful webpage!
I worked on an interface circuit for what sounded very much like a wideband oxygen sensor for a US OEM quite some time ago.
Very delicate little design, had me pulling my hair out for a couple of months...
A lot. They help maintain a 14.7 to 1 air to fuel ratio. If O2 is high engine is considered to be running lean so ECU tells injection to add more fuel and the opposite if running rich. Adjustments to the short and long term fuel trims show the ECU trying to maintain the ratio.
Gummed up MAF or unmetered air throw things out of whack
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.
Very basic ,,, generates a voltage because of the difference between the air on the outside of the sensor and the gases inside the exhaust pipe. volts tell the computer what it needs to do. The AF ratio is an average.
By measuring the oxygen content of the exhaust they can infer the air / fuel ratio being burned in the engine.
This will cause adjustments in the engine operating parameters by the Engine Control Module [Computer] (ECM). It also plays a role in catalytic converter operation ... cats work at a certain elevated temperature, the computer can add oxygen to the A/F mix to increase the Exhaust Gas Temperature which heats up the Cat, then reduce the oxygen content once the cat is working properly to reduce emissions and increase fuel economy and power when needed.
Bad O2 sensor will clearly affect fuel economy and emissions, neither for the better.
Yes as said above 02 sensors typically are 1 in front in behind the cat to look at the unburned oxygen levels, but not always. There are heated O2s and plain O2 sensors. Don't replace the wrong sensor or you could fail an emissions test. The VECI sticker under the hood will tell you what emission components your vehicle will have.