My check engine light come on off and on in a

The O2 sensor is designed to produce voltage based on an electro chemical reaction. It uses oxygen as a catilyst. The oxygen in the sensor combines with the bimetalic cell. Like a battery does with acid and lead/zink plates.
The more oxygen the lower the voltage is and the lower the oxygen the higher the number 9it works the opposite way if its a europeen Luft sensor). any way the O2 sensor values are easily squed if there is any contamination or high milage will also damage an O2 sensor. Like a battery the cell plates may become shorted and or dirty and work at times and not at others.
I hope this helps.
Roy