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Air Fuel Ratio Sensor - How It Works

By R&D
Published on December 21st, 2023

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Air-fuel ratio is a critical parameter in combustion engines as it determines the efficiency and performance of the engine. The proper balance between air and fuel ensures optimal combustion, leading to maximum power output and minimal emissions.

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Air Fuel Ratio Sensor

The Air-Fuel Ratio (AFR) sensor, also known as an Oxygen Sensor (O2S), is a key component in modern combustion engines. It measures the oxygen content in the exhaust gases and provides feedback to the Engine Control Module (ECM) to adjust the fuel injection for optimal combustion.

The primary purpose of the AFR sensor is to monitor oxygen density in the exhaust gas and provide the information to the ECM. The ECM uses this information to adjust the amount of fuel in the air-fuel mixture. By constantly analyzing the oxygen level in the exhaust gases, the sensor enables the ECM to adjust the fuel delivery, ensuring the engine operates at the ideal air-fuel ratio for optimal performance and emission control.


The air-fuel ratio is the ratio of the mass of air to the mass of fuel in the combustion mixture. It directly influences the combustion process, affecting power output, fuel efficiency, and emissions. Achieving the correct air-fuel ratio is crucial for the engine to operate efficiently and meet emission regulations.

The ideal stoichiometric ratio is the chemically correct ratio of air and fuel that allows for complete combustion without any excess air or fuel. For gasoline engines, the stoichiometric ratio is approximately 14.7:1 (14.7 parts of air to 1 part of fuel by mass). Diesel engines have a higher stoichiometric ratio, typically 14.5:1 to 16:1.

A rich air-fuel mixture contains an excess of fuel compared to the stoichiometric ratio, while a lean mixture has less fuel. Rich mixtures can improve engine power at the cost of increased fuel consumption and emissions. Lean mixtures, on the other hand, can improve fuel efficiency but may lead to reduced power and increased nitrogen oxide (NOx) emissions.


As previously discussed, an AFR sensor measures the oxygen content in the vehicle’s exhaust gases. It is also referred to as an oxygen sensor as it detects the presence of oxygen in the exhaust stream.

Traditional oxygen sensors, also known as narrowband sensors, provide a binary signal indicating whether the air-fuel mixture is rich or lean. On the other hand, AFR ensors provide continuous and precise measurements of the air-fuel ratio, allowing for more accurate fuel control and better engine performance.

An AFR sensor typically consists of a sensing element, a reference electrode, a heater element, and a protective housing. The sensing element, usually made of zirconia, generates an electrical voltage based on the oxygen concentration. The reference electrode provides a stable reference voltage, while the heater element ensures the sensor operates at the required temperature for accurate measurements.

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The structure of the Air Fuel Ratio Sensor

How does it works?

The AFR sensor operates based on the principle of the Nernst equation, which relates the oxygen concentration to the voltage output. The sensor compares the oxygen content in the exhaust gases to that of the ambient air and generates a voltage signal proportional to the difference. This signal is then used by the ECM to adjust the fuel injection accordingly.

For more details, the wide-band O2 sensor has two parts; the reference cell and the pump cell. The ECM's AFR sensor circuit tries to maintain a perfect air/fuel ratio in a special chamber by controlling its current. The AFR sensor uses electronic circuits to control the current in the pump cell. In cases where there is insufficient fuel in the air/fuel mixture, the voltage in the pump cell circuit temporarily decreases, and the ECM quickly adjusts the current to maintain a specific voltage or ratio in the chamber. The pump cell then removes extra oxygen through the gap using the current in the circuit. The ECM detects this current and adjusts the injector to add more fuel.

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The operating range of the Air Fuel Ratio Sensor

The heating element in the AFR Sensor ensures that the sensor reaches and maintains its operating temperature. This is essential for accurate measurements, as the sensor's performance is temperature-dependent. The heater element also helps to accelerate the sensor's response time.

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Types of AFR Sensors

There are two main types of AFR sensors: narrowband and wideband. The narrowband sensor provides a binary signal indicating whether the mixture is rich or lean, while wideband sensor provides continuous and precise air-fuel ratio measurements.

Narrowband sensors

Narrowband sensors are the traditional type of AFR sensors. They are designed to operate around the stoichiometric ratio and provide a voltage output that switches between high and low levels, indicating rich or lean conditions. These sensors are limited in their ability to accurately measure air-fuel ratios outside the stoichiometric range.

Wideband sensors

Wideband sensors, also known as broadband or universal sensors, offer a broader measurement range and higher accuracy compared to narrowband sensors. They can accurately measure air-fuel ratios both above and below the stoichiometric ratio, providing valuable feedback for precise fuel control and optimization.

Symptoms of the faulty AFR Sensor

The symptoms of a faulty AFR sensor include:

  • Malfunction Indicator Lamp (MIL) illumination
  • Poor fuel economy
  • Rough idle
  • Engine misfire
  • Difficulty starting
  • Hesitation or stalling
  • Increased emissions

How much does it cost to replace an AFR Sensor?

The price of replacing an Air Fuel Ratio (AFR) sensor differs based on the specific make and model of your vehicle. Generally, replacing an oxygen sensor is a relatively straightforward task unless it poses challenges in terms of accessibility. When done at a repair shop, the labor cost for replacing an oxygen sensor typically ranges from $50 to $250. However, it's worth noting that the expense of replacing an AFR sensor may exceed that of an oxygen sensor.


In this article, we explored the importance of the air-fuel ratio in combustion engines and introduced the AFR Sensor. We reviewed the working principle, types, and construction of AFR sensors, and the differences between narrowband and wideband sensors.

AFR sensors play a crucial role in modern combustion engines by providing accurate feedback to the ECU, enabling precise fuel control and optimization. With their ability to measure air-fuel ratios across a wide range, these sensors contribute to improved engine performance, fuel efficiency, and reduced emissions, making them essential components in today's automotive industry.

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