An oxygen sensor is a necessary component of a car's emissions system. All new cars and many older cars have electronic fuel injection systems, in which a computer regulates the amount of fuel delivered to the engine. The computer communicates with sensors throughout the system to determine how much fuel to deliver to the engine, and how frequently.

The oxygen sensor is usually located in the exhaust manifold. One end of the oxygen sensor detects the oxygen levels in the exhaust flow, and the other end connects to wiring that relays the information to the computer. The computer then uses the sensor readings to ensure that the engine is being given the right amount of fuel. Too much or too little fuel will change the readings from the oxygen sensor, which will prompt the computer to readjust the amounts of fuel being delivered to the engine.

An oxygen sensor will fail periodically. When the sensor malfunctions, important feedback about engine performance will be lost. As a result, the computer that runs the electronic fuel injection system will have no idea how much fuel to deliver to the engine.

An oxygen sensor will always have a mileage rating, which indicates the duration that the oxygen sensor is expected to last. There are several ways to find this information. A car's owner's manual or shop manual should state the expected lifespan of its oxygen sensor. If these books are not available, the dealership can look up the expected lifespan of an oxygen sensor for a specific car. Many aftermarket auto parts stores have the same information. In general, the oxygen sensor in an older car will last approximately 30,000 miles (about 48,280 km), while the oxygen sensor in a newer car can be expected to last 60,000 miles (96,561 km) or more.

Once you know the mileage rating for the oxygen sensor in your car, it is a good idea to keep records of when maintenance is done; if you know when the sensor was last replaced, you'll know when it should be replaced again. Replacing your oxygen sensor regularly maintains your gas mileage, prevents other related car troubles, and helps prevent failed emissions tests due to malfunctioning oxygen sensors.