Lambda sensor: Problems, Testing, Replacement (user:rde-groups CONTAINSANY 'XVIP_AMC_UK' AND user:rde-groups CONTAINSANY 'XHTW_Wholesaler')
Lambda sensor: Problems, Testing, Replacement
 
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Lambda sensor

To make the subject of “Lambda Sensors” more intelligible and to simplify the process of inspecting them in the everyday work of an automotive garage, we show you here the construction and function of a lambda sensor and explain a number of ways of testing them. This technical video gives you practical information and demonstrates how to professionally exchange or replace the lambda sensor. It also cites some typical defects in lambda sensors.

 

Usually, the function of the oxygen sensor is tested during the routine exhaust emissions test. Since it is subject to a certain amount of wear, however, it should be checked for perfect function regularly (approx. every 18.750 miles ) – within the context of a regular service, for example.

 

Important note:
The following technical information and practical tips were created by HELLA to support professional automotive garages in their daily work. The information presented here on this website should only be used by trained automotive technicians taking the respective safety regulations and country-specific legislation and practice into account.

 
 
 

Lambda sensor function

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

Optimum combustion is required if an optimum conversion rate is to be ensured for catalytic converters. 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 mix). This optimum ratio 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 volume supplied:

λ = amount of air fed : theoretical air amount = 14,7 kg : 14,7 kg = 1

The principle of the oxygen sensor is based on a comparative measurement of oxygen content. This means that the residual oxygen content of the exhaust gas (approx. 0.3–3 %) is compared with the oxygen content of ambient air (approx. 20.8 %).

If the residual oxygen content of the exhaust gas is 3 % (lean mixture), a voltage of 0.1 V is produced as a result of the difference to the oxygen content of the ambient air. If the residual oxygen content is less than 3 % (rich mixture) the probe voltage increases in relation to the increased difference to 0.9 V.

The residual oxygen content is measured with different oxygen sensors.
 
 
 
 
Lambda sensor Lambda sensor: Function of the lambda sensor
 

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Measurement using the probe voltage output

This probe comprises a finger-shaped, hollow zirconium dioxide ceramic. The special feature of this solid electrolyte is that it is permeable for oxygen ions from a temperature of around 300 °C. Both sides of this ceramic are covered with a thin porous platinum layer which serves as an electrode. The exhaust gas flows along the outside of the ceramic, the interior is filled with reference air.

Thanks to the characteristic of the ceramic, the difference in oxygen concentration on the two sides leads to oxygen ion migration which in turn generates a voltage. This voltage is used as a signal for the control unit which alters the composition of the air/fuel mixture depending on the residual oxygen content.

This process – measuring the residual oxygen content and making the mixture richer or leaner – is repeated several times a second so that a suitable stoichiometric mixture ( = 1) is produced.
 
 
 

Measurement using probe resistance

With this kind of probe, the ceramic element is made of titanium dioxide – using multi-layer thick-film technology. Titanium dioxide has the property of changing its resistance proportional to the concentration of oxygen in the exhaust gas. If the oxygen share is high (lean mixture λ > 1) it is less conductive, if the oxygen content is low (rich mixture λ < 1) it becomes more conductive. This probe doesn't need reference air, but it has to be supplied with a voltage of 5 V via a combination of resistors. The signal required for the control unit is produced through the drop in voltage at the resistors.

Both measuring cells are mounted in a similar housing. A protective pipe prevents damage to the measuring cells which project into the exhaust gas flow.

Oxygen sensor heating:
The first oxygen sensors were not heated and thus had to be installed near the engine to enable them to reach their working temperature as quickly as possible. These days, oxygen sensors are fitted with probe heating, which allows the probes to be installed away from the engine.

Advantage: they are no longer exposed to a high thermal load. Thanks to the probe heating they reach operating temperature within a very short time, which keeps the period where the oxygen sensor control is not active down to a minimum. Excessive cooling during idling, when the exhaust gas temperature is not very high, is prevented. Heated oxygen sensors have a shorter response time which has a positive effect on the regulating speed.
 
 
 

Broadband oxygen sensors

The oxygen sensor indicates a rich or lean mixture in the range λ = 1. The broadband oxygen probe provides the possibility of measuring an exact air ratio in the lean (λ > 1) and in the rich (λ < 1) ranges. It provides an exact electrical signal and can thus regulate any reference values – e.g. in diesel engines, petrol engines with lean concepts, gas engines and gasheated boilers. Like a conventional probe, the broadband oxygen sensor is based on reference air. In addition, it has a second electrochemical cell: the pump cell.

Exhaust gas passes through a small hole in the pump cell into the measuring space, the diffusion gap. In order to set the air ratio, the oxygen concentration here is compared with the oxygen concentration of the reference air. A voltage is applied to the pump cell in order to obtain a measurable signal for the control unit. Through this voltage, the oxygen can be pumped out of the exhaust gas into or out of the diffusion gap. The control unit regulates the pump voltage in such a way that the composition of the exhaust gas in the diffusion gap is constant at λ = 1. If the mixture is too lean oxygen is pumped out through the pump cell. This results in a positive pump current. If the mixture is rich, oxygen is pumped in from the reference air. This results in a negative pump current. If λ = 1 in the diffusion gap no oxygen is transported at all, the pumping current is zero. This pumping current is evaluated by the control unit, provides it with the air ratio and thus information about the air/fuel mixture.

 
 
 
 

Using several oxygen sensors

 
Since the introduction of EOBD the function of the catalytic converter has also had to be monitored. An additional oxygen sensor is installed behind the catalytic converter for this purpose. This is used to determine the oxygen storage capacity of the catalytic converter.

The function of the post-cat probe is the same as that of the pre-cat probe. The amplitudes of the oxygen sensors are compared in the control unit. The voltage amplitudes of the post-catalytic probe are very small on account of the oxygen storage ability of the catalytic converter. If the storage capacity of the catalytic converter falls, the voltage amplitudes of the post-cat probe increase due to the increased oxygen content.

The height of the amplitudes produced at the postcat probe depend on the momentary storage capacity of the catalytic converter which vary with load and speed. For this reason the load state and speed are taken into account when the amplitudes are compared. If the voltage amplitudes of both probes are still approximately the same, the storage capacity of the catalytic converter has been reached, e.g. due to ageing.
 
 
 

 
 
 

Diagnosis and testing lambda sensors

Vehicles which have a self-diagnosis system can recognise faults in the control cycle and store them in the fault store. This is usually indicated by the engine warning light coming on. The fault code can be read out using a diagnosis unit in order to diagnose the fault. However, older systems are not in a position to establish whether this fault is due to a faulty component or a faulty cable, for example. In this case further tests have to be carried out by the mechanic.

Within the course of EOBD, monitoring of oxygen sensors was extended to the following points:

  • closed wire,
  • stand-by operation,
  • short-circuit to control unit ground,
  • short-circuit to plus,
  • cable breakage and ageing of oxygen sensor.

The control unit uses the form of signal frequency to diagnose the oxygen sensor signals. For this, the control unit calculates the following data:

  • The maximum and minimum sensor voltage values recognised,
  • the time between positive and negative flank,
  • oxygen sensor control setting parameters for rich and lean,
  • regulation threshold for lambda regulation,
  • probe voltage and period duration.

How are maximum and minimum probe voltage determined?
When the engine is started up, all old max./min. values in the control unit are deleted. During driving, minimum and maximum values are formed within a given load/speed range predefined for diagnosis.

Calculation of the time between positive and negative flank.
If the regulation threshold is exceeded by the probe voltage, time measurement between the positive and negative flanks begins. If the regulation threshold is short of the probe voltage, time measurement stops. The time between the beginning and end of time measurement is measured by a counter.

Recognising an aged or poisoned oxygen sensor.
If the probe is very old or has been poisoned by fuel additives, for example, this has an effect on the probe signal. The probe signal is compared with a stored signal image. A slow probe is recognised as a fault through the signal duration period, for example.
 
 
 
 

Lambda sensor testing: Oscilloscope, multimeter, oxygen sensor tester, exhaust emissions measuring device

A visual inspection should always be carried out before every test to make sure the cable and connector are not damaged. The exhaust gas system must be leak-proof. We recommend the use of an adapter cable for connecting the measuring devices.

It must also be noted that the oxygen sensor control is not active during some operating modes, e.g. during a cold start until the operating temperature has been reached as well as at full load.

 
 
 

Testing the lambda sensor with the exhaust emissions tester

One of the quickest and easiest tests is measurement using a four gas exhaust emissions measuring device.

The test is carried out in the same way as the prescribed exhaust emissions test (AU). With the engine at operating temperature secondary air is added as a disturbance variable by removing a hose. The change in composition of the exhaust gas causes a change in the lambda value calculated and displayed by the exhaust emissions tester. From a certain value onwards the fuel induction system has to recognise this and settle this within a given time (60 seconds as with the AU). When the disturbance variable is removed, the lambda value has to be settled back to the original value.

The disturbance variable specifications and lambda values of the manufacturer should always be taken into account.

This test can only be used to establish whether or not the oxygen sensor control is working. An electrical test is not possible. With this method there is the danger that modern engine management systems control the air/fuel mixture through exact load recording in such a way that λ = 1 even if the oxygen sensor control is not working.
 
 
 

Testing the lambda sensor with the multimeter

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

Multimeters with a small internal resistance (usually with analogue devices) place too great a load on the oxygen sensor signal and can cause this to collapse. On account of the quickly changing voltage the signal can be best represented using an analogue device.

The multimeter is connected in parallel to the signal cable (black cable, refer to circuit diagram) of the oxygen sensor. The measuring range of the multimeter is set to 1 or 2 volt. After the engine has been started a value between 0.4-0.6 volt (reference voltage) appears on the display. When the operating temperature of the engine or the oxygen sensor has been reached, the steady voltage begins to alternate between 0.1 and 0.9 volt.

To achieve a perfect measuring result the engine should be kept at a speed of approx. 2,500 rpm. This guarantees that the operating temperature of the probe is reached even when systems with non-heated oxygen sensors are being tested. If the temperature of the exhaust gas is too low during idling, the non-heated probe could cool down and not produce any signal at all.
 
 
 

Testing the lambda sensor with the oscilloscope

The oxygen sensor signal is best represented using the oscilloscope. As with the multimeter, the basic requirement when using the oscilloscope is that the engine or oxygen sensor are at operating temperature.

The oscilloscope is connected to the signal cable. The measuring range to be set depends on the oscilloscope used. If the device has automatic signal detection this should be used. Set a voltage range of 1-5 volt and a time of 1-2 seconds using manual adjustment.

Engine speed should again be approx. 2,500 rpm.

The AC voltage appears as a sinus wave on the display. The following parameters can be evaluated using this signal:

  • The amplitude height (maximum and minimum voltage 0.1-0.9 volt),
  • response time and period (frequency approx. 0.5-4 Hz, in other words fi to 4 times per second).
 
 
 

Testing the lambda sensor with the oxygen sensor tester

Various manufacturers offer special oxygen sensor testers for testing purposes. With this device the function of the oxygen sensor is displayed by LEDs.

As with the multimeter and oscilloscope, connection is to the probe signal cable. As soon as the probe has reached operating temperature and starts to work, the LEDs light up alternately – depending on the air/fuel mixture and voltage curve (0.1–0.9 volt) of the probe.

All the details given here for measuring device settings for voltage measurement refer to zirconium dioxide probes (voltage leap probes). In the case of titanium dioxide probes the voltage measuring range to be set changes to 0-10 volt, the measured voltages change between 0.1--5 volt.
 
 
 

Test the state of the protective pipe

Manufacturer's information must always be taken into account. Alongside the electronic test the state of the protective pipe over the probe element can provide clues about the functional ability:
 
 
 

The protective pipe is full of soot

  • Engine is running with air/fuel mixture too rich.

The probe should be replaced and the reason for the rich mixture
eliminated to prevent the new probe becoming full of soot.

 
 
 
 
Shiny deposits on the protective pipe

  • Leaded fuel is being used. The lead destroys the probe element.

Das Blei zerstört das Sondenelement. Die Sonde muss erneuert und der Katalysator überprüft werden. Bleihaltigen Kraftstoff durch bleifreien Kraftstoff ersetzen.
 
 
 
 

Bright (white or grey) deposits on the protective pipe

  • The engine is burning oil, additional additives in the fuel.

The probe has to be replaced and the cause for the oil burning be eliminated.

 
 
 
 

Unprofessional installation

 

Unprofessional installation can damage the oxygen sensor to such an extent that perfect functioning is no longer guaranteed. The prescribed special tool must be used for installation and care must be taken that the correct torque is used.

 

 
 
 
 

Testing the lambda sensor heating

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

To do this, separate the oxygen sensor connector. Use the ohmmeter to measure the resistance on the two heating element cables at the oxygen sensor. This should be between 2 and 14 Ohm. Use the voltmeter to measure the voltage supply on the vehicle side. A voltage of > 10.5 volt (on-board voltage) has to be present.
 

Various connection possibilities and cable colours

 
 
 

 

 
 
 

Lambda sensor problems: Typical faults

There are a number of typical oxygen sensor faults that occur very frequently. The following list shows diagnosed faults and their causes:
 

Effects of failure

Failure of the lambda sensor may result in the following error indications:

  • High fuel consumption
  • Poor engine performance
  • High exhaust emissions (emissions inspection)
  • Illumination of engine Malfunction Indicator Lamp
  • Storage of error code

 

Failure may be due to a variety of reasons:

  • Internal and external short circuits
  • Ground fault / power supply lacking
  • Overheating
  • Deposits / soiling
  • Mechanical damage
  • Use of leaded fuel / additives
 
 
 

 

 
 
 

Changing the lambda sensor: Clip

 
 
 
 

Replacement of the lambda sensor

Replacing the Lambda probe incl. installation and removal.

 

02:42 min

 
 
 

The following points must be observed when installing the new probe:

If an oxygen sensor is replaced, the following points must be
observed when installing the new probe:

  • Only use the prescribed tool for dismantling and installation.
  • Check the thread in the exhaust system for damage.
  • Only use the grease provided or special oxygen sensor grease.
  • Avoid allowing the probe measuring element to come into contact with water, oil, grease, cleaning and rust-treatment agents.
  • Observe the tightening torques specified by the vehicle and lambda sensor manufacturers.
  • Note the torque of 40-52 Nm for M18x1.5 threads.
  • When laying the connection cable make sure this does not come into contact with hot or movable objects and is not laid over sharp edges.
  • Lay the connection cable of the new oxygen sensor according to the pattern of the originally installed probe as far as possible.
  • Make sure the connection cable has enough play to stop it tearing off during vibration and movement in the exhaust system.
  • Instruct your customers not to use any metal-based additives or leaded fuel.
  • Never use an oxygen sensor that has been dropped on the floor or damaged in any way.