Oxygen sensors, also known as O2 or lambda sensors, are placed before, in, or after the catalytic converter. Some vehicles have four O2 sensors. The O2 sensor monitors the oxygen content in the vehicle’s exhaust. It sends signals to the PCM to maintain a 14.7 to 1 (stoichiometric ratio) air-fuel ratio. This ratio of air-fuel makes the engine run smooth, efficient, and in the end, pollute less.
Upstream O2 Sensor. The O2 sensor before the catalytic converter is called an upstream O2 sensor. It monitors the combustion chamber efficiency.
Downstream O2 Sensor. The O2 sensor located in or after the catalytic converter is called a downstream O2 sensor. It checks catalytic converter efficiency.