The O2 sensor should be somewhere AFTER the air intake. It tells the car's computer whether the gas/air mixture is too lean or too rich.
My 02 sensor is mounted to the part of the exhaust housing the catalytic converter. Try locating a Honda Civic forum. There's tons out there and they could tell you the answer you're seeking within minutes.
I don't think cleaning an 02 sensor does anything but make it clean. New O2 sensors for a Civic should be cheap and easy to find at a local store.
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