What is an oxygen sensor?
An oxygen sensor is a measuring probe for determining the oxygen content of the exhaust gas. Since the amount of oxygen in the exhaust gas indicates precisely how complete the combustion of the air-fuel mixture in the cylinders is, it is also the best starting point for controlling the air-fuel ratio.

How do they work?
The oxygen sensor is strategically located in the exhaust system. The outside surface of the ceramic measuring tube protrudes into the exhaust gas flow, and the inner surface is in contact with the outside air. A voltage is generated at the interface which is proportional to the relationship between residual oxygen in the exhaust gas and that of the surrounding air. When this relationship changes, so does the voltage. This voltage is processed by an electronic control unit (ECU) into a control signal for influencing the air-fuel mixture through controllable fuel injection or carburetor systems. The exhaust gas composition is thus always maintained at that level which permits effective after-treatment by the vehicle's catalytic converter.

Why should I change my oxygen sensors?
A new oxygen sensor can improve driveability and lengthen engine life. Ultimately, you achieve a smoother running engine that is more fuel efficient. Feedback from the oxygen sensor provides closed-loop control of the injected quantity of fuel for optimum air-fuel mixture...enabling virtually complete combustion to take place. By providing closed-loop control of the mixture, it becomes possible to use three-way catalytic converters to achieve the maximum reduction in exhaust gas emissions.

How often should I change them?
Your vehicle manufacturer recommends a specific mileage at which your oxygen sensor(s) should be changed. However, contaminants in gas and certain atmospheric conditions can drastically shorten the life of any oxygen sensor, therefore they should be checked with every periodic maintenance and changed if necessary.
Where are the sensors located on my vehicle?
Every vehicle has the oxygen sensors located in a slightly different place. Typical locations include the exhaust manifold & before & after the catalytic converter.