How a lambda sensor works

What is a lambda sensor?

In simple terms, a lambda sensor measures the amount of oxygen in your exhaust fumes, to ensure your engine is burning fuel correctly.

We’ll dive a little deeper into exactly how and why in a moment. We’ll also answer a few other questions, such as ‘how do you test a lambda sensor?’ and ‘which lambda sensor should I choose?’

Keeping your engine running as it should, while limiting harmful emissions

Lambda sensors were introduced in 1977 to improve the efficiency of car engines. Fitted in both petrol and diesel vehicles, they help to reduce the number of harmful emissions, primarily gases such as carbon monoxide, and pollutants, produced by your car.

The sensors are designed to work within government exhaust gas legislation. Due to the role they play in the operation of your car, they are also widely known as oxygen sensors or O2 sensors.

The science behind your lambda sensor operation

Air to fuel ratio

When your car burns petrol or diesel, it mixes it with air to provide the most efficient operation of your engine.

This ratio of air to fuel is known as the stoichiometric ratio. Or, and much easier to pronounce, the lambda ratio. Lambda is a Greek letter, represented by λ.

Running rich

When your fuel is running rich, it means there’s not as much air in the mixture as there should be. With fuel that runs rich, there is excess, unburned fuel. Unburned fuel creates pollution, the stuff we’re trying to avoid.

Running lean

When there’s too much air in your fuel mix, it creates a lean fuel mixture. A lean fuel mix tends to produce more nitrogen-oxide pollutants. It can also cause poor engine performance and possible engine damage.

How does the lambda sensor work to correct your fuel mix?

Your car will have at least one sensor in your exhaust system, to measure the amount of oxygen in the fumes after the fuel’s combustion.

In modern cars, there are often 2 sensors. The first is directly after the engine and before the catalytic converter. The second is placed after the catalytic converter to monitor the overall operation. It also checks that your cat is doing is its job correctly.

Your lambda sensor converts the amount of oxygen present in your exhaust into an electric signal and sends the signal to the computer that controls your engine operation.

The ECU (engine control unit) processes the readings and sends the information back to the engine. The engine then makes compensations of how to mix fuel and air to get the ratio back to where it needs to be.

The voltage your sensor creates operates between 0.1V and 0.9V. A 0.1V reading translates into a fuel mixture that’s running lean, and a 0.9V reading, one that’s running lean. The optimum voltage for a perfect mix is 0.45V.

How often does a lambda sensor need replacing?

Due to the nature of their operation and their position in an extremely hot and dirty environment, your lambda sensor will wear out over time.

Several things can affect your sensors lifespan, but typically, it should last anywhere between 50k and 100k miles.

Early sensors didn’t have a heating element. They required the exhaust temperature to reach a specific heat to operate. Modern sensors are fitted with a heating element, taking a lot of the pressure off the sensor. These newer sensors have a much longer lifespan.

Your sensor should be checked periodically, to ensure its correct operation.

How to tell if your lambda sensor isn’t working properly

  • Your engine performance will suffer—often misfiring, cutting out or not starting at all
  • When your engine is idling, or just ticking over, it will feel rough and lumpy compared to normal
  • Engine performance is poor
  • Fuel consumption is higher than normal
  • Your car has failed an emission test
  • The engine warning light will illuminate on your dashboard

How to test a lambda sensor

There are several ways to test a lambda sensor.

1. Checking your Lambda sensor with an exhaust tester

A quick and easy way to measure your lambda sensor’s performance is with a four-gas emission analyser. This is carried out in the same way as your emissions test. The lambda value is calculated from the changing exhaust gas composition over 60 seconds, to check that the maintained ratio is always operating at 1.

Checking your lambda sensor with a multimeter

You should use only a high-impedance multimeter with a digital display. The multimeter should be connected parallel to the signal line of the sensor and set to 1V or 2V. When you start your engine, a reading between 0.4–0.6V should appear. Once the engine is up to temperature, the reading should alternate between 0.1–0.9V. The ideal engine speed for the best measurements should be at 2,500rpm.

Checking your lambda sensor with an oscilloscope

Connect the oscilloscope to the signal line. Set a voltage range of 1–5V and a time setting of 1–2 seconds and again, run the engine at 2,500rpm. The amplitude height of the waveform will deliver your maximum and minimum voltage (0.1–0.9V) and the response time and period duration will show the frequency (0.5–4Hz).

Checking your lambda sensor with a Lambda sensor tester

You can buy a device purely designed for the measurement of your Lambda sensor. As you would with an oscilloscope or multimeter, connect the tester to the signal line, and when you achieve the correct temperature, your reading will be displayed using the LED scale.

Always replace your sensor, like-for-like

Given there are hundreds of sensors available, you could be asking, “What lambda sensor do I need?”

You should always check the manufacturer’s recommendation, as there are different types of sensor and you need the correct option for your ECU.

When it comes to replacing your sensor, here are a few tips for a clean and correct installation:

  • Clean the thread in the exhaust thoroughly.
  • Apply only the supplied and correct type of grease to the sensor threads. Do not grease the sensor nose.
  • Only tighten the sensor to the prescribed torque setting. Use a torque wrench with a suitable lambda sensor socket. Over-torquing is dangerous to any sensor with a heater element, as it can crack the internal ceramic and cause the sensor to fail.