Oxygen sensor - How It Works
To make the subject of oxygen sensors more easily understood and simplify testing in day-to-day garage work, we would like to present the function and the different testing possibilities with the oxygen sensor in this
issue.
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.
What is the oxygen sensor for?
As a result of more stringent laws governing the reduction of exhaust
emissions from motor vehicles, exhaust gas treatment techniques have
also been improved. Optimum combustion is necessary to guarantee an
optimum conversion rate of the catalytic converter. This is achieved when
the air/fuel mixture is composed of 14.7 kg of air to 1 kg of fuel (stoichiometric mixture). This optimum mixture is described by the Greek letter
(lambda). Lambda expresses the air ratio between the theoretical air requirement and the actual amount of air fed:
Structure and function of the
oxygen sensor
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.
Measurement using the probe voltage output (voltage leap probe)
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 ceramicare 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 (resistance leap probe)
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
In the case of V and boxer engines with double-flow exhaust systems two oxygen sensors are usually used. This means each cylinder bank has its own control cycle that can be used to regulate the air/fuel mixture. In the meantime, however, one oxygen sensor is being installed for individual cylinder groups in in-line engines, too (e.g. for cylinders 1-3 and 4-6). Up to eight oxygen sensors are used for large twelve-cylinder engines using the latest technology.
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.
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.
Testing the oxygen sensor using an 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 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 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. 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 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.
Measurement using the probe voltage output (voltage leap probe)
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 ceramicare 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 (resistance leap probe)
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
Using several oxygen sensors
In the case of V and boxer engines with double-flow exhaust systems two oxygen sensors are usually used. This means each cylinder bank has its own control cycle that can be used to regulate the air/fuel mixture. In the meantime, however, one oxygen sensor is being installed for individual cylinder groups in in-line engines, too (e.g. for cylinders 1-3 and 4-6). Up to eight oxygen sensors are used for large twelve-cylinder engines using the latest technology.
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.
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.
Testing the oxygen sensor using an 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 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 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. 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. The probe has to be replaced and the catalytic converter checked. Use lead-free fuel instead of leaded fuel.
- 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 oxygen 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.
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