An oxygen sensor (O2 sensor) is a device fitted into the exhaust system of your vehicle to detect the amount of unburned oxygen that is present in the exhaust gases coming from your engine. During combustion, your engine burns a mixture of air and fuel inside the combustion chambers. When that air/fuel ratio is out of balance, excess amounts of unburned fuel or unburned oxygen escape through the exhaust.
To correct these conditions, a vehicle has two to four oxygen sensors mounted near the exhaust manifold or on the exhaust pipe near the catalytic converter. These sensors read the amount of oxygen present in the exhaust and send data to the engine’s computer (the Engine Control Module, or ECM). The ECM analyzes this data and adjusts the air/fuel mixture for optimal performance and fuel economy.