Updated at: 22-03-2022 - By: Lucas

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What may be the difference between sensors that measure oxygen upstream and those that measure oxygen downstream?

In general, an upstream O2 sensor is put prior to the catalytic converter (between the manifold and the CAT), while a downstream O2 sensor is installed after the catalytic converter (typically prior to the muffler), as the name suggests..

However, the ECU processes the data from these two sensors differently so that the engine can perform at its best. Upstream and downstream O2 sensors are compared here in this succinct and thorough comparison.

Oxygen sensors are installed in every car that has an emissions system (O2 sensors). The number of oxygen sensors in a vehicle is determined by the engine’s cylinder count and other factors.

Two upstream and two downstream O2 sensors are standard equipment in nearly every car.

Functions of Oxygen Sensors

Functions of Oxygen Sensors

It is the primary role of O2 sensors to measure the amount of exhaust fuel. Using this information, the engine control unit (ECU) can better regulate the flow of fuel, air, and lubricant to the various parts of the engine.

Oxygen sensors located both upstream and downstream serve the same purpose.

The Engine Control Unit (ECU) receives readings from both upstream and downstream OS sensors.

See also: Oil Pressure Sensor Symptoms

Because an O2 sensor maintains a precise air-fuel ratio, it reduces exhaust emissions by ensuring that the engine does not receive more or less air and fuel than it should.

Upstream Vs Downstream Oxygen Sensors

Downstream Vs Upstream O2 Sensor

Let’s take a closer look at the differences and similarities between these two kinds of oxygen sensors so we can draw some conclusions.

What is an Upstream O2 Sensor?

Upstream O2 sensors are situated between the exhaust manifold and the CAT, therefore they are referred to as “upstream” O2 sensors. When the engine is running, they assist us determine how much pollution is being emitted into the atmosphere.

It is possible for upstream sensors to detect raw, unburned gas from the combustion chambers and transmit the information via ECU.

What is a Downstream O2 Sensor?

“Downstream O2 sensor” located between the catalytic converter and the muffler; its primary duty is to measure the contaminants in the exhaust fumes emitted by the CAT.

An error code or sign may be generated if the ECU detects the same pollutant levels as those found in the upstream sensor readings. This is because it appears that the catalytic converter is not doing anything to lower the pollutants in exhaust emissions.

What Is The Difference Between Upstream And Downstream O2 Sensors

Oxygen sensors can be classified as “upstream” or “downstream” based on where they are located in relation to the catheter arm. The positions of the “upstream” and “downstream” O2 sensors are clearly defined by the information provided above.

Because of this, it is not possible to employ upstream and downstream oxygen sensors interchangeably. The installation of upstream and downstream O2 sensors necessitates careful consideration of the pros and cons of each option.

In addition, the pattern of the thread should always be scrutinized. It is possible to utilize O2 sensors with the same thread pattern as upstream and downstream interchangeably if they have different thread patterns.


In the exhaust system, oxygen sensors are used to monitor the amount of pollutants and contaminants in the exhaust. “Upstream” and “Downstream” sensors are based on the location of an oxygen sensor.

The comparison between an upstream and downstream O2 sensor is based on the sensor’s location; if the sensor is located before or after the CAT, it is an upstream sensor.

O2 sensors can fail, and the check engine light will come on, as well as a variety of strange symptoms.