If it is malfunctioning it can affect fuel mileage.
the 02 sensor has nothing to do with gas mileage. it sipley is a sensor that reads the oxygen level in your exhaust for emissions.
A faulty oxygen sensor can make a car smell like gas. Oxygen and gasoline mixing wrong can cause excessive pollution. Replacing the oxygen sensor can correct this issue, and improve the gas mileage in the car.
causes very poor gas mileage
Check ingine light and your gas mileage get worse.
no but it affects how well your car runs and gas mileage
Absolutely. Not just on Subaru but every car built that has an oxygen sensor. And they all do.
If you removed ALL emissions equipment, the question would be hard to answer, in theory anyway. Removing individual equipment such as your Catalytic Converter would improve exahaust flow and improve gas mileage. Removing your Oxygen Sensor would most likely decrease gas mileage because the Oxygen Sensor helps control the air/fuel mixture.
No, does not work.
There is NO Oxygen Sensor in the Gas Tank.....
If the one you are replacing is defective then a new one will help your fuel mileage. If on the other hand if it is working as it should then a new one will not help.
If the old oxygen sensor is no longer functioning properly and you replace it with a new one you will certainly save quite a bit of fuel.
Generally a Bad Oxygen Sensor will cause the engine to run too Rich...too much gas mixed with not enough air. This will decrease Gas Mileage, and lower the engine's performance. Also the Check Engine light will come on!