Oxygen sensors in a car measure the amount of unburned oxygen present in the exhaust. They send the information to the car's computer, which adjusts the fuel-to-air ratio to ensure the highest fuel efficiency for the car while emitting the lowest amount of pollution, according to Smog Tips.
Most cars have between one and four oxygen sensors that, depending on the type, are placed along the exhaust system from just before the catalytic converter to the exhaust manifold, states Smog Tips. Some cars have sensors located just before and just after the catalytic converter that send signals to the computer indicating how effective the catalytic converter is at burning off excess hydrocarbons and carbon monoxide in the exhaust, states AA1Car.
Together with the catalytic converter and the car's computer, properly functioning oxygen sensors are a critical part of meeting federal guidelines for emissions and passing an emissions test. They also affect the car's mileage, because the computer constantly adjusts whether the car runs rich, on a ratio of more gas to air, or lean, on more air to gas, depending on the signals sent by the oxygen sensors. A faulty oxygen sensor or a dirty catalytic converter will cause the car to use more gas and will eventually make the check engine light come on, according to AutoTap Express DIY.