Part of the car's emission system, the exhaust oxygen sensor functions to monitor the levels of oxygen in the exhaust system. The exhaust system includes the exhaust manifold, which is fixed to the cylinder head and takes the exhaust fumes from the engine's cylinders into the exhaust pipe; a catalytic converter, which sits between the exhaust manifold and the rear exhaust box, and changes the harmful emissions into water vapour and carbon dioxide; the rear exhaust box which acts as an exhaust silencer; and the exhaust pipe through which the exhaust travels, and the means by which the fumes reach the tail pipe where they finally leave the vehicle.
Exhaust oxygen sensors are located in the exhaust pipe, positioned in front of or behind the catalytic converter. The exhaust oxygen sensors situated at the front measure how much oxygen is in the gas as it leaves the combustion chamber.
The exhaust oxygen sensors positioned behind the catalytic converter measure how much oxygen is in the gas as it exits the catalytic converter. If there is too much oxygen in the exhaust gas, the gas is referred to as being too 'lean'. If there's too little oxygen in the gas, it is referred to as being too 'rich'. The fuel to air ratio needs to be exactly right in order for the conditions to be perfect for optimum combustion. If there is not enough air in the mix this causes a surplus of fuel that did not get burnt in the combustion process which is not good, as this unburned fuel is a pollutant. If there is too much air in the mix, this produces more nitrogen-oxide pollutants in the gas leaving the system and can also lead to reduced performance and damage to engine components. Based on data gathered by the exhaust oxygen sensors, the ECU makes adjustments to the amount of fuel flowing into the engine to ensure the oxygen levels in the fuel/air mix are correct. This process ensures emissions are kept as low as possible. The overall result of this ensures the engine is working to its optimum, providing more power and fuel efficiency.
As is often the case with electrical components, the exhaust oxygen sensor can fail because of damage caused by corrosion, extreme heat or it can simply wear out. A faulty exhaust oxygen sensor won't be able to accurately measure the level of oxygen in the exhaust system so the ECU won't have the right data to make the correct adjustments to manage how much fuel is entering the engine. This leads to poor engine performance and significantly reduces fuel efficiency, while increasing the car's emission output. When replacing this sensor, installing a good quality exhaust oxygen sensor will provide you with a part that is compatible with the make and model of your vehicle.