Oxygen Sensor

What is an Oxygen Sensor or Lambda Probe?

The Oxygen Sensor, also known as Lambda Probe, is located in the vehicle exhaust, and its main function is to analyze the amount of oxygen present in the gases released by the engine.

What is the Oxygen Sensor or Lambda Probe for?

This sensor is for collecting fuel burn information and sending it to the ECM, or engine control module.









Internal combustion engines (Otto Cycle, Diesel, or CNG) can only function if there is oxygen, fuel, and heat (combustion or burning is an exothermic reaction, that is, it occurs from the inside out). Without these elements, it is not possible to obtain the internal explosion necessary for the engine operation.

The big challenge, however, is getting the balance between fuel and oxidizer – in this case, oxygen –, which results in the stoichiometric mixture. That is where the oxygen sensor comes in, measuring the unburned oxygen resulted from the engine combustion. If the mixture is lean (more oxygen than fuel) or rich (less oxygen than fuel), the sensor sends an electric signal (in millivolts) to the electronic injection management unit (ECMCU)– which, based on the information from the sensor, will regulate the fuel mixture, injecting more or less fuel into the combustion chamber. This allows for better engine performance, fuel saving, and lower emissions.









Example: For pure gasoline to burn completely, we need 14.7 parts of air for 1 part of fuel. This ratio must be changed according to different conditions, whether environmental (temperature, pressure, humidity, etc.) or related to the vehicle’s own operation (RPM, engine temperature, desired power variation, among others).


Oxygen Sensor or Lambda Probe?

The correct and comprehensive name for all types of this product is OXYGEN SENSOR. It accurately measures the oxygen (O2) present in combustion, regardless of which fuel is used.

When the mixture is rich (too much fuel), the voltage generated by the sensor is high (900 millivolts). At this moment, the ECM stops injecting fuel and the mixture becomes lean (too much oxygen). Then the sensor informs the ECU with a low voltage (50 millivolts) and, at this moment, the ECU injects more fuel in the mixture.  

See, on the graph below, the reason why the Greek letter lambda (λ) was adopted to name the sensor.

Switching – Oxygen Sensor / Lambda Probe




What Is the Lambda Factor?

The letter is also used to define the lambda factor (λ), which corresponds to the equivalence between the actual air-fuel ratio (happening in the vehicle at that time) and the ideal or stoichiometric ratio for a mixture.


Lambda factor (λ) = actual air-fuel ratio

ideal air-fuel ratio

<alinhar os itens acima como se vê na referência a seguir (tirada do texto em português): >

Ideal air-fuel ratio

  • Gasoline: 14.7: 1 (14.7 parts air to 1 part gasoline)
  • Ethanol: 9.0:1 (9.0 parts air to 1 part ethanol)
  • Diesel: 15.2:1 (15.2 parts air to 1 part diesel)

Thus, we can conclude that when a mixture has more air than specified in the list above, λ > 1 – that is, the mixture is lean. When the amount of air is below the specified, λ < 1 – that is, the mixture is rich.


How Does the Oxygen Sensor Work?


What Is the Oxygen Sensor Made of?

The oxygen sensor is composed of an internal ceramic material called zirconium dioxide and a porous platinum coating, all protected by a metal casing. Its performance is based on changing the properties of the ceramic material at high temperatures, allowing the diffusion of oxygen in the air.


It operates according to the difference in oxygen concentration between the exhaust gas and outside air, generating a voltage of 50 mV to 900 mV.

The sensor has a limitation: to start operating, it must be heated to about 300 °C (575 °F).

Older sensors were only heated by the exhaust gases, so it was necessary to wait several minutes before they could work properly. Currently, oxygen sensors have heating resistors that allow for the heating in up to 10 seconds, even when the exhaust gases are at a low temperature.


How Many Oxygen Sensors Does the Vehicle have?

One to four sensors, depending on the engine type or car age. It is usually located in the exhaust manifold near the engine and before the catalyst converter. In this position, the sensor controls the mixture of fuel and oxygen. It can also be found in the exhaust pipe after the catalyst, measuring the condition of the catalytic converter.



What Types of Oxygen Sensors Are Found in Vehicles?

1 – Thimble Type 

They are available in models of one to four wires, depending on the construction project. Due to environmental legislations, newer vehicles only use oxygen sensors with an internal heater, commonly found in four-wire thimble sensors, as the heating starts to work in approximately 40 seconds after ignition.



2 – Planar Type

This four-wire type has a new design that promotes a 15-second heating of the sensor, thus faster than the thimble type. Consequently, it starts monitoring the air-fuel ratio much faster as well.




3 –  Wideband Sensor Type – Four Wires

Also known as the Air / Fuel ratio sensor, this type delivers more accurate measures, seeking the optimal ratio. It can monitor how rich or lean the mixture is, differently from the thimble and planar types. It is mostly used in Asian makes, like Honda, Nissan, and Toyota.




4 –   Wideband Sensor Type – Five Wires

Like the Air / Fuel ratio sensor, it can monitor the ratio in detail according to the vehicle condition.





How Does the Wideband Sensor Work?

The wideband sensor, also known as A / F ratio sensor, was designed to provide a linear output signal for vehicles that should fit the Euro 3 standard.

This sensor allows for more precise and gradual mixture control and offers a faster response.

See its characteristic curve compared to the usual oxygen sensor’s:

The usual oxygen sensor above 300 °C (575 °F) generates a voltage between 0.2V and 0.9V (or 200 to 900 millivolts), thus a binary system that changes from low voltage (lean mixture) to high voltage (rich mixture) – that is, a lambda factor 1 (λ=1).

The wideband sensor, when above 650 °C (1200 °F), is a voltage generator as well, but it is almost linear for mixtures with a lambda factor between 0.75V and 1.5V. This means that its response is proportionate to the oxygen concentration.

Wideband sensors are manufactured in three different configurations:
1. With five wires, two cells, and a CLOSED diffusion chamber.
2. With five wires, two cells, and an OPENED diffusion chamber.
3. With four wires and only one cell.


Before continuing, let’s learn about the Nernst cell, the main component of the oxygen sensor:



irconium ceramic element allows the passage of oxygen ions from one side to another. On one side, there is atmospheric air with 21% oxygen and, on the opposite side, we find the exhaust gases with little to no oxygen. This movement of ions generates a voltage of up to 1 volt.


The wideband sensor uses two Nernst cells: one as a measuring cell and another to inject oxygen (oxygen pump). If the difference in oxygen concentration generates voltage, so, when a voltage is applied, it generates an ion flow, that is, an ionic current.


The measuring cell (sensor 1) is the same we find in a usual oxygen sensor. Its outer side is in contact with the exhaust gases, while its inner side is in contact with the other cell – the oxygen injection cell (sensor 2) –, creating a diffusion chamber in between them. This second cell is the one in contact with the atmosphere.


Type 1 

With two cells and a closed diffusion chamber, the ECU (electronic control unit) regulates the voltage applied to the injection cell (2) in order to keep the signal of the measuring cell (1) always at 0.45 V. The voltage applied in cell 2 ranges between 1.7 V (for rich mixtures) and 3.3 V (for lean mixtures).


Type 2

With two cells and an open diffusion chamber, this type presents some differences: the measuring cell (1) is inside the sensor and in contact with the reference air; the injection cell (2) is on the outside, in contact with the exhaust gases, just like the diffusion chamber, which has a cavity for access to exhaust gases.

Here is an example of a rich mixture in the exhaust. As the diffusion chamber becomes slightly rich, it generates a voltage increase in the measuring cell.

In the ECU, there is a circuit that compares this voltage with a 0.45V reference. It generates a negative voltage in order to inject oxygen. As no oxygen is present in the rich exhaust gases, it is generated by an electrochemical reaction taking place on the thin layer of the platinum electrode (exhaust side), which splits the oxygen ions from carbon monoxide and water present in the exhaust gases.

This oxygen is injected into the diffusion chamber until a stoichiometric condition is established. When the mixture has λ = 1, the injection current is null. When there is a lean mixture, the circuit generates a positive current and removes oxygen from the diffusion chamber.



With only one cell, it is known as AF sensor. In this case, the sensor has only one Nernst cell with an atmospheric air reference cavity, very similar to the usual oxygen or lambda sensor. The difference here is that there is a special diffusion chamber, which limits the ion flow of oxygen when a voltage is applied between the electrodes.



Here Is How It Works

The ECU applies 3.3 V to the internal electrode and 3 V to the external electrode, so there is between them a difference of electrical potential of 300 mV, which slightly forces an oxygen ion injection from the exhaust gases side into the air reference chamber.

When the gases are at a very rich mixture (A/F < 14.7) – that is, with no oxygen, the Nernst cell apprehends the high difference of oxygen concentration and generates a maximum voltage between the electrodes, as it injects oxygen from the reference chamber out to the exhaust gases side. This movement of ions is opposite to that forced by the 300 mV, which means that there is a negative current between the electrodes, so that the reference voltage at the ECU drops below the 3.3 V.

In the opposite case, when the exhaust mixture is too lean (A/F > 14.7), there is an excess of oxygen on the external side, which is in favor of the 300 mV forced injection, facilitating the oxygen- ion flow and generating a positive current. The reference voltage in the ECU rises above the 3.3 V.

When the gases are at stoichiometric balance, the 300 mV forced injection cancels the flow generated by the Nernst cell and there is no ion flow – and thus no electrical current. The reference voltage stays at 3.3 V


How Good Is the Oxygen Sensor?

Follow the seven MTE-THOMSON steps to make this assessment.




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