The lambda sensor measures the oxygen content in the exhaust gas and ensures that the engine is provided with an optimal mixture of air and fuel. If the lambda sensor is defective, symptoms such as increased fuel consumption or poor engine performance often become apparent. In our guide, you can find out everything about lambda sensors and receive instructions on how to replace the component.

Function of the lambda sensor in the car

Modern cars have at least one lambda sensor, which is usually screwed into the exhaust system directly behind the exhaust manifold. Here, the sensor continuously determines the residual oxygen content in the exhaust gas by comparing the oxygen it contains with the oxygen content of the ambient air.

The lambda reading is transmitted to the engine’s control unit in the form of a voltage. Based on this voltage, the engine control unit can detect whether the fuel-air ratio is optimally set and adjust the mixture composition for the engine accordingly. Modern vehicles often have a second diagnostic probe that measures the residual oxygen content behind the catalytic converter to check its condition.

Change or clean the lambda sensor?

If the lambda sensor is faulty, many people ask themselves: Change or clean? Both are possible as long as the lambda sensor is only dirty. Cleaning the sensor can sometimes save costs for a replacement part. However, the lambda sensor must be removed and reinstalled for cleaning, just as it is for replacement, so that the expense is not reduced.

Often lambda sensors are contaminated by combustion residues or soot. This can limit the correct function. When cleaning the sensor, the dirt as well as blockages of the openings are removed. The cleaning of the lambda sensor should be done with utmost care in order not to damage the component. The same applies when removing and installing the sensor.

Symptoms of a defect in the lambda sensor

If the lambda sensor is defective, incorrect measured values are made available to the engine control unit, or the signal is missing completely. As a result, the engine control unit can no longer correctly detect the residual oxygen content in the exhaust gas, which means that the fuel-air ratio can no longer be optimally adjusted.

Various symptoms can indicate a defective lambda sensor. For example, the exhaust gas values can deteriorate if the ratio of oxygen and fuel is no longer correct. This may only become apparent during the next exhaust emission test. More obvious signs are a smoke emission from the exhaust or a foul smell of the exhaust gases.

Engine without power or error message

If a lambda sensor fails, the engine’s performance can also drop as a result. Without the signal from the sensor, the engine control system is missing an important parameter. Oxygen and fuel can no longer be optimally adjusted, which means that the engine tends to burn less efficiently. Depending on the model, the car may even jump into emergency mode.

Modern vehicles often detect a defect in the lambda sensor via the on-board diagnosis. In this case, the engine control lamp lights up or the driver is informed of the fault as an error message in the display of the on-board computer. At the same time, the error is entered in the error memory for later inspection by the workshop.

Lambda sensor defective Overview of symptoms:

• Fuel consumption is increased
• Engine power decreases
• Pollutant emissions on the rise
• Smoke coming from the exhaust
• Smell of the exhaust gases is more unpleasant
• Indicator light in the dashboard flashes
• On-board computer gives error message
• Car jumps into the emergency programme

Causes for a defective lambda sensor

The possible causes of a defect in the lambda sensor are manifold. Although the sensor is a robust component, the increasing wear and tear can lead to a defect with high mileage. In the area of the exhaust system, the lambda sensor is exposed to great stresses due to the effects of heat, but also dirt and weather.

Wetness and road salt often cause corrosion of the lambda sensor under the vehicle and thus lead to a defect. The soot particles and aggressive combustion residues contained in the exhaust gas can also cause contamination of the sensor over the years. This sometimes results in an incorrect lambda value, or the lambda sensor fails completely.

Fuel or corrosion of the connections

Possible causes for a defective lambda sensor can also be unprofessional tuning of the engine and unsuitable additives for the fuel. Both lead temporarily to an increased temperature in the exhaust gas and thus to high heat, which damages the lambda sensor. The use of fuel additives can also lead to contamination of the sensor.

In addition, mechanical damage to the diagnostic sensor is also possible, e.g. due to the vehicle being placed on top of it. Corroded connections on the lambda sensor can also be responsible for a failure of the lambda signal. However, it is not always the lambda sensor itself that is defective when symptoms for the failure of the component appear. Defective cables also ensure that the signal is no longer transmitted correctly.

Common causes for a defect in the lambda sensor:

• Wear after long mileage
• Heat in the area of the exhaust system
• Corrosion on the lambda sensor
• Soot and combustion residues clog the probe
• Fuel additives were used
• Damage due to mechanical action
• Connections are corroded
• Lambda sensor cable is defective

Changing the defective lambda sensor

If the lambda sensor is defective, the component should be replaced as soon as possible. Depending on the vehicle model and the defect, continued operation is possible, but not advisable. Optimum regulation of the mixture composition is not possible, which means that the engine does not run perfectly and emits more pollutants.

Continue driving with a defective lambda sensor? Not a good idea. The car will be noticed at the next emissions test at the latest. If the pollutants in the exhaust gas exceed the permissible limits, the vehicle will not pass the main inspection. In addition, the catalytic converter can be damaged if the vehicle is driven permanently with a defective sensor.

Changing the lambda sensor in 4 steps

Replacing the lambda sensor with a new or used spare part is usually done quickly. Expert car mechanics are happy to carry out the replacement of the sensor themselves in order to keep the costs of the repair low.

The exact procedure may vary from vehicle to vehicle and may differ slightly from the following instructions. The best way to carry out the repair is to lift the vehicle onto a lifting platform.

1. Check the lambda sensor: Before replacing the installed lambda sensor with a spare part, it is a good idea to check the sensor. Workshops can check the lambda sensor by connecting an appropriate diagnostic device. However, the specified voltage can also be determined with a multimeter. In addition, the cables and connections should be checked. It is not uncommon for a damaged connection or a marten bite to be to blame for the failure.
2. Remove the defective sensor: If it is certain that the lambda sensor is defective, you can begin with the removal of the component. To do this, first disconnect the connector plug and then remove the sensor from the thread. Old sensors are often very tight, so that there is a risk of tearing off the thread. Heating the exhaust system slightly or using appropriate means to loosen it can sometimes prove helpful. If the old part is to be disposed of anyway, a ring spanner can be slid over the cable after cutting off the plug.
3. Prepare the new lambda sensor: After removing the defective sensor, the intact replacement part can be installed. Before this, the new lambda sensor is compared with the removed part and checked for fit. Professionals lubricate the thread with a special high-temperature lubricant before installation, which makes the installed part easier to screw in and also easier to loosen. Then the component is first carefully screwed in by hand and then tightened with a suitable tool.
4. Complete the repair: In the final step of the repair, the lambda sensor cable is reconnected by connecting the connectors together. Any corrosion on the connector on the vehicle side should be removed beforehand, e.g. with suitable cleaning agents for contacts. If an error code has been stored in the error memory, it can now be deleted with an appropriate device.

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