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Old 05-20-2022, 11:54 AM   #9 (permalink)
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A narrow band O2 sensor uses 2 dissimilar metals that chemically react with each other, when above 660 degrees F, and in the lack of oxygen. They are capable of outputting well over 2 volts (personally observed). As the sample chamber is diluted with oxygen, the output voltage decreases. In other words, they work similarly to a battery.

Starting rich, the voltage is around 1 volt. As the AFR goes lean, the output doesn't change that much. Then around the target AFR (14.7:1) the voltage begins to drop rapidly, and then tapers off. A new sensor may have enough output current to be able to use something like posted. As it ages, the output current decreases, even if the voltage remains reliable (again think in terms of a variable battery).
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