Updated at: 23-03-2022 - By: Lucas

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Almost everything in a car is critical, from the engine and gearbox to the drivetrain and transmission components. You’d notice a decrease in performance and maybe an increase in fuel/oil consumption if any component were to fail. What is the purpose of a downstream O2 sensor?

One of the most important functions of the catalytic converter’s downstream sensor is to monitor the amount of pollutants being emitted from the exhaust after it passes through it.

Upstream and downstream sensors are among the most essential of the many sensors found in autos.

An O2 Sensor’s primary function is to monitor the concentration of oxygen in the blood.

“Oxygen Sensor” refers to the downstream O2 sensor type. Every new car with an emissions system has this feature. The number of oxygen sensors (also known as O2 sensors) in a modern vehicle normally consists of two upstream and two downstream sensors.

A common practice is to add O2 sensors in the exhaust system to measure the amount of dangerous pollutants being released from the engine’s combustion chamber to the exhaust’s tailpipe.

What Does a Downstream O2 Sensor Do?

What Does A Downstream O2 Sensor Do

In order to limit emissions pollution, the ECU uses the data (signals) collected by O2 sensors to make adjustments to fuel and oil delivery.

If the ECU detects a high amount of pollutants in the exhaust, it may activate the check engine light as a means of alerting the driver to the fact that the catalytic converter isn’t performing as well as it should be.

‘Pollutants,’ in this context, refer to the amount of unburned oxygen in the exhaust emissions leaving the vehicle’s tailpipe. There appears to be a direct correlation between the amount of unburned oxygen in the emissions and the environmental impact.

If the CAT’s emissions contain additional contaminants, it’s clear that the CAT isn’t working properly. A catalytic converter “converts” hazardous pollutants from the combustion chamber of the engine into less damaging gases before they are emitted out the exhaust.

Can a Car Drive Without Downstream Sensors?

Your check engine light will come on if the downstream sensor is bad, but your car will still run. A downstream sensor is not mandatory, but it is recommended. It’s easy to replace downstream sensors because they’re affordable and don’t take long.


A downstream O2 sensor, then, is what? Catalytic converter (CAT) emissions are monitored by a sensor that measures the amount of unburned gas in the exhaust stream.

You can tell if your CAT is likely to fail if your car’s downstream sensors detect more unburnt gas in the exhaust. Unlike downstream O2 sensors, which are installed after the catalytic converter, upstream O2 sensors are installed before the catalytic converter (behind the exhaust manifold).