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Vehicles with computerized engine control systems rely on data from several sensors to maintain optimal engine performance, emissions, and other important functionality. The sensors must provide accurate information, or the vehicle can experience drivability issues, increased fuel consumption, and emission failures.

One of the primary sensors in this somewhat complicated system is the Oxygen Sensor. It is often referred to as the O2 Sensor or the Lambda Sensor, which is its technical name.

Although oxygen sensors were first used in 1976, the state of California introduced legislation in 1980 that toughened emission rules, and this spurred the growth in O2 sensor usage. In 1981, federal emission laws made oxygen sensors virtually mandatory on all cars and light trucks. Today’s vehicles are now subject to more stringent OBD-II regulations, and many vehicles are now equipped with multiple oxygen sensors – some with as many as four!

The sensor is typically mounted in the exhaust manifold, and it monitors how much unburned oxygen is in the exhaust as it exits the engine. Monitoring oxygen levels in the exhaust is a way of gauging the fuel mixture. It tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen). The engine then adjusts automatically to run at peak performance.

A lot of factors can affect the relative richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, airflow, and engine load. There are other sensors to monitor these factors too, but the oxygen sensor is the primary monitor for what happens with the fuel mixture. Consequently, any problems with this sensor can cause problems for the entire fuel system.

Fuel Mixture Feedback Control Loop

Your vehicle’s on-board computer uses the oxygen sensor input to regulate the fuel mixture; this process is referred to as the fuel "feedback control loop." The computer takes its cues from the sensor and responds by modifying the fuel mixture to an ideal level. This generates a corresponding change in the sensor reading. This is referred to as "closed-loop" operation because the computer is using the oxygen sensor’s input to normalize the fuel mixture. The result is a constant flip-flop, back and forth from rich to lean, which allows the catalytic converter to operate at peak efficiency while keeping the average overall fuel mixture in proper balance to minimize emissions.

If no signal is received from the sensor, as would be the case when a cold engine is first started (or the sensor fails), the computer orders a fixed (unchanging) rich fuel mix.