Originally Posted by
lionelhutz
To keep it simple, the O2 goes below 0.5V when the engine is lean and it goes above 0.5V when the engine is running rich. The switching point is a lambda of 1.0 (complete combustion) or what people call an AFR of 14.7:1. During times like decel and wide open throttle, the ECM will go open loop and pick a lower or higher AFR which drives the sensor to a continuous high or low voltage. Remember, an AFR below 14.7 is rich which is a high voltage and an AFR above 14.7 is lean which is a low voltage.
When closed loop, the ECM continuously varies the fuel. It will first begin to remove a little fuel until the O2 voltage goes low. Then, it will add a little fuel until the O2 voltage goes rich. I suggest you look up the explanation of a PI or PID control loop to get a better idea of what it's doing.
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