What Is an Oxygen Sensor in a Car?


Knowing what your car's oxygen sensor does and why it's so important can make the cost of having it replaced a little less painful. A history of the sensor, an understanding of it does and a few facts about the device will help you understand why it's so important.


  • The oxygen sensor, or "O2" sensor, was invented to compliment fuel injection. In the 1970s, when fuel-injected engines were becoming available, the Engine Control Unit (ECU) needed to know how much air or gas was not being burned after it left the engine. The oxygen sensor was developed to measure the exhaust of the engine so that the ECU could better direct the injection of fuel into the engine.


  • The oxygen sensor serves a very simple purpose--it measures the level of oxygen, nitrogen and fuel leaving the engine cylinders. If the sensor detects a high level of fuel, the ECU reduces the amount of fuel entering the engine. If the oxygen sensor detects too much air, the ECU delivers more fuel to the engine.


  • As you can imagine, a sensor that monitors engine exhaust is constantly exposed to fumes. If a car is burning oil or running "rich," particles and chemicals form residues on the oxygen sensor, causing it to fail. At this point, the "check engine" lights up to alert you of the failure.

Expert Insight

  • Oxygen sensors are generally found in the exhaust manifold immediately after the engine in the exhaust line. Most cars have at least two sensors: one in the exhaust manifold, and another in or after the catalytic converter.

Older Cars

  • As cars get older and develop more problems, the need for replacing the oxygen sensors diminishes. If a car is leaking or burning oil, the sensor is ruined from the contaminates in the exhaust. Without fixing the major engine problem, the oxygen sensors will keep failing, costing you more money.

Fun Fact

  • Volvo was the first automotive company to implement an oxygen sensor in their cars. Due to their implementation of the oxygen sensor, modern production cars still use O2 sensors to properly control the air and gas mixture in the engine, and also gives us cleaner air to breathe.

Computer Code

  • Most local automotive stores will read the code from your car when it throws a "check engine" warning. The computer they use can normally tell you what is malfunctioning in your car.

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