Oxygen sensors are typically positioned on either side of the catalytic converter. They measure how much oxygen is in the exhaust system, and in the waste gases. Ideally, there would be none.
What Does An Oxygen Sensor Do?
An oxygen sensor helps to fine-tune the intake process and is often located in the exhaust manifold.
Based on the oxygen levels, a sensor then either adds or reduces fuel in the combustion process. This allows the exhaust to have the right fuel to air mixture.
The engine should use the exact amount of oxygen required to burn the fuel completely. This would mean that no oxygen or fuel would be leftover.
The Engine Control Unit (ECU) of a modern fuel-injected car, analyses the exhaust sensor oxygen reading. Then the ECU can help make sure that the best mix of air to fuel is in the engine cylinder. It does this by changing the amount of fuel being injected.