The role of BMW oxygen sensor is to effectively control fuel emissions for better engine performance and fuel economy. It analyzes the exhaust fumes and deposits produced by the engine to gather data of the air-fuel ratio. If the ratio is off, the sensor sends feedback to the vehicle's engine control system so the fuel mixture can be adjusted accordingly. Common signs of a faulty BMW oxygen sensor are unstable idling speeds and a sudden drop in gas mileage. Heated sensors need to be replaced after 100,00 miles; 50,000 miles for unheated.