Upstream Vs Downstream Oxygen Sensors – Key Differences

There are affiliate links on this article. If you make a purchase through any of the links, I may earn a small commission at no extra cost to you.

Wondering what could be the different between upstream vs downstream oxygen sensors?

Basically, upstream O2 sensor is usually installed before the catalytic converter (between the manifold and the CAT), while a downstream O2 sensor is installed after the catalytic converter – usually before the muffler.

In practice, there’s no difference between these two sensors, but then, the ECU processes their data differently to keep the engine functioning at its best. Here’s a concise and detailed comparison of upstream and downstream O2 sensors.

Every vehicle that has an emissions system also has a set of oxygen sensors (O2 sensors). The number of O2 sensors in a car is dependant on the number of cylinders the engine has, and how the engine is designed to function.

Practically, every car has two to four O2 sensors; 2 upstream sensors and 2 downstream ones.

Functions of Oxygen Sensors

The main function of O2 sensors is to read the amount of fuel in the exhaust. This reading is then sent to the ECU where the computer processes it to determine and adjust fuel, air, and oil delivery within the engine to improve performance.

Upstream and downstream oxygen sensors practically perform the same function.

Both upstream and downstream OS sensors’ readings are sent to the Engine Control Unit (ECU) to regulate air and fuel delivery.

Read Also: Symptoms Of a Bad Oil Pressure Sensor

So, basically, an O2 sensor helps to maintain a perfect air-fuel ratio, which in turn helps to reduce exhaust emissions because the engine won’t receive more or less air and fuel than supposed.

Upstream Vs Downstream Oxygen Sensors

Upstream Vs Downstream Oxygen Sensors

To compare these two types of oxygen sensors, let’s highlight the specific functions and how they work collaboratively.

What is an Upstream O2 Sensor?

It is called the “Upstream” oxygen sensor because it is located before the catalytic converter; upstream O2 sensors sit between the exhaust manifold and the CAT. They help to measure the level of pollutants coming from the engine to the exhaust system.

Upstream sensors can detect raw, unburned gas from the combustion chambers; when such is detected, the information is sent to ECU for further processing.

What is a Downstream O2 Sensor?

A “Downstream” O2 sensor sits between the catalytic converter and the muffler; its core function is to detect the level of pollutants in the emissions released by the CAT, which would go down the exhaust tailpipe.

If the amount of pollutant detected by the downstream sensors is the same as what was detected by the upstream sensors, then the ECU may trigger a catalytic converter inefficiency error code/sign – since it appeared like the converter didn’t do anything to reduce the pollutants in the emissions sent from the engine.

What Is The Difference Between Upstream And Downstream O2 Sensors

With the explanations above, it is easy to detect the difference between these two types of oxygen sensors; one sits at the front of the CAT while the other sits after the CAT; the position of O2 sensors is the major difference between “upstream” O2 sensors and “downstream” O2 sensors.

However, upstream and downstream oxygen sensors are pretty designed differently and they cannot be used interchangeably. You need to carefully select between upstream and downstream O2 sensors for installation at their respective positions.

Furthermore, there’s another thing to always check out for, and that’s the thread pattern. O2 sensors typically have different thread patterns; if you have O2 sensors with the same thread patterns, you can use them interchangeably as upstream or downstream.

Read Also: Why Is My Check Engine Light On?


Oxygen sensors perform one particular function, which is to read and detect the containment and pollutants in the emissions sent through the exhaust system. The position of an oxygen sensor is what makes it an “Upstream” sensor or “Downstream” sensor.

So, summarily, the comparison of upstream vs downstream O2 sensor centers on the position of the sensor; when the sensor is before the CAT, it’s an upstream sensor, and when it’s situated after the CAT, it’s downstream.

If any of the O2 sensors get bad, the ECU would trigger the check engine light to illuminate, and you may experience other unusual signs too.

Scroll to Top