Lambda sensor - cause of failure, testing and troubleshooting

The lambda sensor determines the remaining oxygen content in the exhaust emission, and provides the engine control with an electric signal to control the air-fuel ratio. Scroll through this page and find out about the variants, how they work, test methods, and important information on the correct replacement of lambda sensors.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.


Optimal combustion is required in order to ensure an ideal conversion rate of the catalytic converter. In the case of a gasoline engine, this is achieved with an air-fuel ratio of 14.7 kg air to 1 kg fuel (stoichiometric mixture). This optimum mixture is designated by the Greek letter λ (lambda). Lambda is used to express the air ratio between the theoretical air requirement and the actual air flow supplied:


λ = supplied air flow : theoretical air flow = 14.7 kg : 14.7 kg = 1

The lambda sensor principle is based on an oxygen comparison measurement. This means that the remaining oxygen content of the exhaust emission (approx. 0.3 – 3 %) is compared with the oxygen content of the ambient air (approx. 20.8 %).


If the remaining oxygen content of the exhaust emission is 3 % (lean mixture), there is a voltage of 0.1 V due to the difference compared with the oxygen content of the ambient air.


If the remaining oxygen content is less than 3 % (rich mixture), the sensor voltage rises to 0.9 V in proportion to the increased difference. The remaining oxygen content is measured using various lambda sensors.


The function of the lambda sensor is usually checked during the routine exhaust emissions test. However, since it is subject to a certain amount of wear, it should be checked at regular intervals to ensure that it is working properly (approx. every 30,000 km) – e.g. as part of the inspections.


The tightening of laws to reduce vehicle exhaust emissions has been followed by an improvement in the technology for exhaust aftertreatment.

Voltage jump probe

This probe consists of a finger-shaped, hollow zirconium dioxide ceramic element. The characteristic feature of this solid electrolyte is that it is penetrable for oxygen ions at a temperature higher than approx. 300 °C. Both sides of the ceramic are coated with a thin, porous layer of platinum which works as an electrode. The exhaust emissions flow past the exterior of the ceramic element, while the interior is filled with reference air.


The properties of the ceramic element mean that the differing oxygen concentration on both sides causes a migration of oxygen ions, which in turn generates a voltage. This voltage is used as a signal for the control unit, which changes the air-fuel ratio depending on the residual oxygen content of the exhaust emissions.


This process of measuring the remaining oxygen content and making the mixture richer or leaner is repeated several times per second, generating a demand-based mixture ( λ = 1).

Resistance jump probe

On this type of sensor, the ceramic element is made from titanium dioxide using multi-layer, thick-film technology. One property of titanium dioxide is that its resistance changes in proportion to the oxygen concentration in the exhaust emissions. With a higher oxygen content (lean mixture λ > 1) it is less conductive, with a lower oxygen content (rich mixture λ < 1) it becomes more conductive. This probe does not require any reference air, but must be supplied with a voltage of 5 V by the control unit via a combination of resistors. The signal required for the control unit is generated by the drop in voltage at the resistors.


Both measuring cells are assembled in a similar housing. A protective tube prevents damage to the measuring cells protruding into the exhaust flow.

Lambda sensor heating

The first lambda sensors were not heated, and so had to be installed near the engine in order to reach their operating temperature as quickly as possible. Nowadays, lambda sensors are equipped with sensor heating. This means that the probes can also be installed away from the engine.


They are no longer exposed to the high thermal load. The sensor heating enables them to reach their operating temperature within a short period, keeping the time in which the lambda control is not active to a minimum. Excessive cooling is prevented during idle mode, when the exhaust gas temperature is not as high. Heated lambda sensors have a shorter response time, which has a positive effect on the control speed.

Broadband lambda sensor

The lambda sensor displays a rich or lean mixture in the range of λ = 1. The broadband lambda sensor offers the option to measure a precise air ratio in both the lean (λ > 1) and rich (λ < 1) ranges. It delivers an exact electrical signal and can therefore control any reference values – e.g. for diesel engines, gasoline engines with lean concepts, gas engines, and gas heaters. Like a conventional probe, the broadband lambda sensor is built up with reference air. It also has a second electrochemical cell: The pump cell.


Exhaust emissions enter into the measurement area through a small hole in the pump cell, known as the diffusion gap. In order to set the air ratio (λ), the oxygen concentration here is compared with the oxygen concentration of the reference air. In order to receive a measurable signal for the control unit, a voltage is applied at the pump cell. With this voltage, the oxygen can be pumped from the exhaust emissions into or out of the diffusion gap. The control unit regulates the pump voltage in such a way that the ratio of the gas is constantly at λ = 1 in the diffusion gap. If the mixture is lean, oxygen is pumped outward by the pump cell. A positive pump current is the result of this. If the mixture is rich, oxygen is pumped inward from the reference air. A negative pump current is the result of this. At λ = 1 in the diffusion gap, no oxygen is transported; the pump current is zero. This pump current is evaluated by the control unit, providing the air ratio and thus information about the air-fuel ratio.

Use of multiple lambda sensors

Use of multiple lambda sensors

Since the introduction of the EOBD, the function of the catalytic converter must also be monitored. An additional lambda sensor is installed behind the catalytic converter for this. This is used to determine the ability of the catalytic converter to store oxygen.


The function of the probe downstream of the catalytic converter is the same as the upstream probe. The amplitudes of the lambda sensors are compared in the control unit. The voltage amplitudes of the downstream probe are very small due to the ability of the catalytic converter to store oxygen. The lower the storage capacity of the catalytic converter, the higher the voltage amplitudes of the downstream probe due to the increased oxygen content.


The heights of the amplitudes at the downstream probe are dependent on the actual storage capacity of the catalytic converter, which varies depending on the load and speed. The load condition and speed are therefore taken into consideration when comparing the probe amplitudes. If the voltage amplitudes of both probes are still roughly the same, the storage capacity of the catalytic converter has been reached, e.g. through aging.


A faulty lambda sensor can cause the following symptoms:

  • High fuel consumption
  • Poor engine performance
  • High exhaust emissions
  • Engine indicator lamp comes on
  • Error code is stored


There are several reasons why a failure may occur:

  • Internal and external short circuits
  • No ground / voltage supply
  • Overheating
  • Deposits / contamination
  • Mechanical damage
  • Use of leaded fuel / additives

There are a range of typical lambda sensor faults that occur frequently. The following list shows the causes behind diagnosed faults:

Unheated probes

Diagnosed faults Cause
Protective tube or probe body clogged with oil residues Unburned oil has found its way into the exhaust system, e.g. due to faulty piston rings or valve stem seals
False air intake, lack of reference air Probe installed incorrectly, reference air opening blocked
Damage due to overheating Temperatures above 950 °C due to incorrect ignition point or valve play
Poor connection at the plug contacts Oxidation
Interrupted cable connections Poorly routed cables, abrasion points, rodent bites
Lack of ground connection Oxidation, corrosion at the exhaust system
Mechanical damage Excessive tightening torque
Chemical aging Short routes very often
Lead deposits Use of leaded fuel



Vehicles that are equipped with self-diagnostics can detect faults occurring in the control circuit and store them in the fault memory. This is usually displayed through the engine indicator lamp. The fault memory can then be read out with a diagnostic unit for fault diagnostics. However, older systems are not able to determine whether this fault relates to a defective component or e.g. a cable fault. In this case, further tests must be carried out by the mechanic.


As part of the EOBD, the lambda sensor monitoring has been expanded to include the following points:

  • Open circuit,
  • Operational readiness,
  • Short circuit to control unit ground,
  • Short circuit to positive
  • Cable break and lambda sensor aging.

In order to diagnose the lambda sensor signals, the control unit uses the form of the signal frequency.


For this, the control unit calculates the following data:

  • The maximum and minimum detected sensor voltage value,
  • The time between the positive and negative flank,
  • Lambda controller regulating variable according to rich and lean,
  • Control threshold of the lambda control,
  • Probe voltage and period duration.


When starting the engine, all old max/min values in the control unit are deleted. During operation, the min/max values are shown in a load/speed range specified for the diagnostics.


If the probe voltage exceeds the control threshold, the time measurement between the positive and negative flank begins. If the probe voltage falls short of the control threshold, the time measurement stops. The time period between the start and end of the time measurement is measured by a counter.


If the probe is severely aged or contaminated, e.g. through fuel additives, this has an impact on the probe signal. The probe signal is compared with a stored signal pattern. A slow probe is detected as a fault, e.g. via the period duration of the signal.


As a basic principle, a visual inspection should be carried out before each check to ensure that there is no damage to the cable or connector. The exhaust system must not have any leaks.


It is recommended to use an adapter cable to connect the measuring device. It must also be ensured that the lambda control is not active during some operating states, e.g. during cold start until the operating temperature is achieved, and when at full load.

Checking the lambda sensor with the exhaust tester

One of the quickest and easiest tests is to measure with the four-gas emission analyser.


The test is carried out in the same way as the prescribed exhaust emissions test. With the engine is at operating temperature, false air is connected as a disturbance variable by removing a hose. Through the changing exhaust gas composition, the lambda value that is calculated and displayed by the exhaust tester also changes. The mixture formation system must detect this from a certain value and adjust it within a certain time (60 seconds, as in the exhaust emissions test). If the disturbance variable is removed, the lambda value must be reduced to the original value.


As a basic principle, the specifications for disturbance variable connection and the lambda values of the manufacturer should be observed.


However, this test can only determine whether the lambda control is working. An electrical test is not possible. With this procedure, there is a risk that modern engine management systems control the mixture through precise load detection so that λ = 1, despite the lambda control not working.

Checking the lambda sensor with the multimeter

Only high-impedance multimeters with digital or analog display should be used for the test.


Multimeters with a low internal resistance (mostly in analog devices) overload the lambda sensor signal and may cause it to break down. Due to the quickly alternating voltage, the signal is best depicted with an analog device.


The multimeter is connected parallel to the signal line (black cable, see circuit diagram) of the lambda sensor. The measuring range of the multimeter is set to 1 V or 2 V. After the engine is started, a value between 0.4 – 0.6 V appears on the display (reference voltage). If the operating temperature of the engine or lambda sensor is reached, the fixed voltage begins to alternate between 0.1 V and 0.9 V.


In order to achieve flawless measuring results, the engine should be kept at a speed of approx. 2,500 rpm. This ensures that the operating temperature of the probe is reached, even in systems with an unheated lambda sensor. If the exhaust gas temperature is not sufficient in idle mode, there is a risk that the unheated probe cools down and a signal is no longer generated.

Checking the lambda sensor with the oscilloscope

The lambda sensor signal is best depicted using the oscilloscope. As for the measurement with the multimeter, a basic prerequisite is that the engine or lambda sensor must be at operating temperature.


The oscilloscope is connected to the signal line. The measuring range to be set is dependent on the oscilloscope used. If the device has automatic signal detection, this should be used. For manual adjustment, set a voltage range of 1 – 5 V and a time setting of 1 – 2 seconds.


The engine speed should once again be approx. 2,500 rpm.


The alternating voltage appears on the display in sinusoidal form. The following parameters can be evaluated at this signal:

  • Amplitude height (maximum and minimum voltage 0.1 – 0.9 V),
  • Response time and period duration (frequency approx. 0.5 – 4 Hz).

Checking the lambda sensor using the lambda sensor tester

Various manufacturers offer special lambda sensor testers for testing. With this device, the function of the lambda sensor is displayed via LEDs.


Like the multimeter and oscilloscope, it is connected to the signal line of the probe. Once the probe has reached operating temperature and begins working, the LEDs start to light up in alternation – depending on the air-fuel ratio and voltage curve (0.1 – 0.9 V) of the probe.


Here, all specifications for the measuring device settings for the voltage measurement relate to zirconium dioxide sensors (voltage jump sensors). For titanium dioxide, the voltage measuring range changes to 0 – 10 V, with the measured voltages alternating between 0.1 – 5 V.

Checking the condition of the protective tube

The manufacturer's specifications must be observed as a basic principle. Alongside the electronic test, the condition of the probe element protective tube may give an indication about the functional capability:


  • Engine running with mixture that is too rich


The probe should be exchanged and the cause for the excessively rich mixture remedied, to prevent the probe from becoming sooted again.


  • Use of leaded fuel


The lead destroys the probe element. The probe must be replaced and the catalytic converter checked. Replace the leaded fuel with unleaded fuel.


  • Engine burns oil, additional additives in the fuel


The probe must be replaced and the cause for the oil burning remedied.


Improper mounting may damage the lambda sensor such that proper functioning cannot be guaranteed. During mounting, the prescribed special tool must be used and the tightening torque observed.


The internal resistance and voltage supply of the heating element can be checked.


For this, disconnect the connector to the lambda sensor. On the lambda-sensor side, use the ohmmeter to measure the resistance at both cables for the heating element. This should be between 2 and 14 ohms. On the vehicle side, use the voltmeter to measure the voltage supply. There must be a voltage > 10.5 V (on-board voltage).

Different connection options and cable colors

Unheated probes

Number of cables Cable colour Connection
1 Black Signal (ground via housing)
2 Black Signal


Heated probes

Number of cables Cable colour Connection
3 Black
2 x white
Signal (ground via housing) of heating element
4 Black
2 x white
Signal, heating element, ground


Titanium dioxide probes

Number of cables Cable colour Connection
4 Red
Heating element (+)
Heating element (-)
Signal (-)
Signal (+)
4 Black
2 x white
Heating element (+)
Heating element (-)
Signal (-)
Signal (+)

(Manufacturer's specifications must be observed)


Replacement of the lambda sensor

Replacement of the lambda sensor inc. removal and installation instructions


02:42 min

The following must be noted during replacement of the lambda sensor

If a lambda sensor is replaced, the following should be noted during mounting of the new probe:

  • During removal and mounting, only use the specific tool intended for this purpose.
  • Check the thread in the exhaust system for damage.
  • Only use the grease supplied, or grease which is specifically intended for lambda sensors.
  • Avoid bringing the probe's measurement element into contact with water, oil, grease, cleaning agents, and rust removal agents.
  • Observe the respective tightening torques specified by the lambda sensor manufacturer and vehicle manufacturer.
  • When routing the connecting cable, ensure that it does not come into contact with hot or mobile objects and is not routed over sharp edges.
  • Route the connecting cable of the new lambda sensor according to the pattern of the originally installed probe as far as possible.
  • Ensure that the connecting cable has sufficient play so that it does not break away from the exhaust system through vibrations and movements.
  • Instruct your customers that they should not use any metal-based additives or fuel containing lead.
  • Never use a lambda sensor which has fallen onto the ground or is damaged.
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