O2 Sensor

The oxygen sensor (or O2 sensor) is the starting point of your vehicle's emission feedback loop.

Oxygen sensors are designed to monitor the amount of oxygen that is in your car's exhaust stream. They are used to check for combustion efficiency and to confirm that the catalytic converter is functioning properly.

These sensors determine whether your engine is running an air/fuel mixture that is too rich or too lean by oxidizing a portion of the oxygen as it comes through the exhaust system. That reading is then turned into a voltage signal that is sent back to the engine control computer, which then adjusts fuel metering and timing to maintain an optimum mixture. This is something that's constantly changing in real-time, according to engine load (i.e. hills), acceleration, engine temperature and warmup period, and other factors.