Think of the A/F sensor as a generator capable of changing polarity. When the fuel mixture is rich (high exhaust oxygen content), the A/F generates current in the negative (-) direction. As the air/fuel mixture gets leaner (less oxygen content), the A/F sensor generates current in the positive (+) direction. At the stoichiometric point, no current is generated.
The detection circuit is always measuring the direction and how much current is being produced. The result is that the ECM knows exactly how rich or lean the mixture is and can adjust the fuel mixture much faster than the oxygen sensor based fuel control system. Therefore, there is no cycling that is normal for a narrow range oxygen sensor system. Instead, A/F sensor output is more even and usually around 3.3 volts.
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