What Is a Lambda Sensor and What Does It Do?

Ellie Dyer-Brown, 23 days ago

5 min read

  • How it works
  • Exhaust
Checking an O2 sensor

WhoCanFixMyCar explains everything you need to know about lambda sensors.

The lambda sensor is also known as the oxygen sensor. It’s a small probe in the exhaust system that helps your car maintain the correct air-to-fuel ratio. 

This guide provides a deep dive into how the lambda sensor works, why it’s important, and why it sometimes needs replacing.


What is a lambda sensor?

What does a lambda sensor do?

How does a lambda sensor work?

How often does a lambda sensor need replacing?

Signs of lambda sensor failure

How to test a lambda sensor

Can I unplug the lambda sensor and still drive?

What is a lambda sensor?

Volvo introduced lambda sensors in the 1970s to measure the amount of oxygen in exhaust gases, and they became a legal requirement throughout Europe in the 1990s. Older cars typically have one sensor between the exhaust manifold and the catalytic converter, whereas newer models have two, with the second one placed behind the cat.

Find out more about how car exhaust systems work.

What does a lambda sensor do?

The lambda sensor detects the amount of oxygen in exhaust gases and sends this information to the ECU, which adjusts the air-fuel ratio for optimised engine performance.

A new oxygen sensor with white background

How does a lambda sensor work?

The sensor converts information about the exhaust gas oxygen level into an electrical signal that the ECU can read. The voltage created by the sensor ranges from 0.1V (indicating the engine is running lean) to 0.9V (indicating it’s running rich). In response to this electric signal, the ECU changes the ratio of air and fuel going into the engine.

The science behind the lambda sensor

Air-to-fuel ratio

Car engines work efficiently by using a mix of fuel and air. The ratio of fuel to air is called the stoichiometric or lambda ratio. Lambda is the word for the eleventh letter of the Greek alphabet, λ, representing scales and balance.

Running lean

When your engine is running lean, it means there is too much air in the fuel mixture. A lean mixture produces more nitrogen-oxide pollutants and often causes poor performance.

Running rich

Running rich means there is not enough air in the air-fuel mixture, creating excess unburnt fuel. Again, this can cause more pollution.

How often does a lambda sensor need replacing?

Lambda sensors last a long time - between 50,000 and 100,000 miles. However, they operate in dirty and hot conditions, which eventually causes them to wear out. Newer sensors have a heating element, so they don’t need the engine to warm up before they start working and tend to last much longer.

oxygen sensor

Signs of lambda sensor failure

The following symptoms could indicate a problem with your car’s lambda sensor:

How to test a lambda sensor

There are a few different ways to test a lambda sensor.

lambda / o2 sensor in car

Using a multimeter

Use a high-impedance multimeter with a digital display screen. Connect it parallel to the signal line of the sensor and set it to one or two volts. When you switch the engine on, you should see a reading between 0.4 and 0.6V; when it gets up to temperature, it should alternate between 0.1V and 0.9V.

The ideal engine speed for this test is 2,500 rpm. If your car is fitted with newer sensors with a heating element, you won’t have to wait for the engine to warm up for an accurate reading.

Using an exhaust tester

You can use a four-gas emission analyser, which is done in the same way as a regular emissions test. The lambda value is worked out from the changing exhaust gas composition over 60 seconds.

Using an oscilloscope

Set a voltage range of one to five volts and a time setting of one to two seconds. Connect the oscilloscope to the signal line and run the engine at 2,500 rpm. The waveform’s amplitude height shows the maximum and minimum voltage (0.1-0.9V), and the response time and period duration show the frequency (0.5-4Hz).

Using a lambda sensor tester

The easiest way to test the lambda sensor is using a device designed explicitly for that purpose. All you need to do is plug it into the signal line, and the device will show a reading on the LED display. If your car is fitted with an old sensor, you’ll have to wait for the engine to get up to temperature before taking a reading.

Can I unplug the lambda sensor and still drive?

We wouldn’t recommend driving without a functional lambda sensor. Your car will produce much higher emissions, leading to an MOT failure the next time it’s tested. The catalytic converter will be at greater risk of getting clogged, which is expensive to fix. You will also have to spend more money on fuel.

Find out how to get the best fuel efficiency from a car.

Are you having trouble with your car’s lambda sensor? WhoCanFixMyCar can help you find a reliable local garage for a repair.

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