General description 
      The lambda sensor, also called lambda probe, measures the level of oxygen in exhaust gases and it’s placed on the engine exhaust. By analyzing the waveforms of operation of the lambda sensor in different modes of engine operation, functioning of the sensor itself can be assessed as well as the functioning of the engine management system on the whole. Sign of a malfunctioning lambda sensor is increased fuel consumption, vehicle dynamics reduction, loss of engine power, erratic idling or incorrect idle speed.

A typical view of the Lambda sensor is shown in fig.1.

Fig. 1

Typical colors of the О2 sensor cables 
Red   – signal (active end)
Gray – sensor’s ground
White (two pieces) – 12V heater power supply

What is an oxygen sensor and how it works

      Petrol engines require an exact air-fuel mixture proportion for a proper operation. The proportion, in which the fuel burns completely and effectively, is called a stoichiometric and is exactly 14.7:1. This means that one part of fuel must be mixed with 14.7 parts of air. In practice, this air-fuel proportion varies depending on the engine operation mode and the mixture formation. Thus the engine is uneconomic.
      The excess air coefficient – L (lambda) characterizes how far is the actual fuel-air mixture from the stoichiometric (14.7:1). This mixture is considered optimal and in this case L = 1. If L < 1, we have a lack of air and the mixture is enriched. When L = 0.85 - 0.95 engine power is increased. If L > 1, there is an excess of air and the mixture is leaned. Engine power drops down when L = 1.05 - 1.3, but the economy rises. At L > 1.3 mixture becomes impossible to ignite and engine misfire occurs. Petrol engines reach their maximum power when a lack of air 5-15% (L = 0.85 - 0.95) is present, and a minimum fuel consumption is achieved with an excess air of 10 – 20% (L = 1.1 – 1.2). 
      Thereby when the engine is working, the proportion L is constantly varying in the range 0.9 - 1.1 and this is the lambda regulation operating range. When the engine warms up to its operating temperature and it’s not loaded (i.e. idling), keeping the equality L = 1 is essential in order the catalytic converter to completely fulfill its purpose and reduce the vehicle’s emissions to a minimum.
      Oxygen sensor is mounted on the exhaust manifold so that exhaust gases can be on the streamline of its working surface. In essence the oxygen sensor is a galvanic current source, which changes its output voltage according to the temperature and the environment oxygen content. Depending on the exhaust gases oxygen concentration, a different output signal appears. Shape of this signal depends on the type of material the sensor is made from. Thus the oxygen sensor reports the onboard controller the amount of oxygen in exhaust gases. Clock edge of the signal between its "high" and "low" state, is negligible and can be ignored. The onboard controller receives signal from the oxygen sensor, compares it with a value stored in its memory and if the signal differs from the optimal for the current mode, it adjusts the fuel injection duration in both directions. Thus, by the implementation of a feedback and a correct operation mode, a maximum fuel economy and minimum harmful gases, is achieved.

Types of oxygen sensors
According to the substance used in their sensitive element, are:

  • Zirconium (zirconium oxide)
  • Titanium (titanium oxide)
  • Wideband

According to their design:

  • Single-wire lambda sensor
  • Two-wire lambda sensor
  • Three-wire lambda sensor
  • Four-wire lambda sensor

      Single-wire lambda sensor was used in the early injection systems with a feedback (lambda regulation). It has only one terminal, which is the signal terminal. Sensor ground is its housing and it connects to the engine ground through the exhaust pipes.
      Two-wire lambda sensor has a separate grounding cable. It was used in the early injection systems with a feedback (lambda regulation) also.
Disadvantage of the single-wire and the two-wire sensors is that their operating temperature range starts at 300 ºC. Sensor will not work and will not produce a signal until this temperature is reached. It was necessary for the sensor to be mounted as close to the engine cylinders as possible in order to heat and wrap from the hottest exhaust gases stream. Process of heating the sensor slows down the regulation process of the onboard controller because of the feedback. In addition, using the exhaust pipe as a signal ground requires sensor’s thread to be coated with a special electrically conductive paste, which increases the possibility of a bad contact in the feedback circuit.
      In the three-wire lambda sensors, is a special heating element inside which is constantly turned on when the engine is working and thus it’s reducing the heating time of the sensor to the working temperature. This allows installation of the sensor on the exhaust manifold, near the catalytic converter. Disadvantage is the need of electrically conductive grease.
In the four-wire oxygen sensors - two of the terminals are the heater terminals and the other two, the signal terminals.

Principle of operation of a zirconium oxygen sensor

      The zirconium oxide oxygen sensor generates an output voltage signal from 40mV¸100mV to 0.7V¸1.0V. Output voltage signal range of a properly functioning oxygen sensor is up to ~950mV.
      When oxygen content in the exhaust is low and the engine runs with an enriched mixture, the sensor will generate а high voltage signal 0.65V¸1V. In case of a high oxygen content in exhaust (lean mixture), the sensor generates a low voltage signal 40mV¸250mV.
      Properly operating oxygen sensor starts to work only if its sensitive element has been heated to a temperature of at least 350 ºC. Then its outcome electrical resistance significantly decreases and it acquires the ability to alter the reference voltage coming from the onboard controller through a resistor with a constant electrical resistance. This reference voltage is 450mV for most of the onboard controllers. Such controller considers the oxygen sensor ready for use only when as a result of the heating of the sensor, it can change the voltage in range greater than ±150mV ~ ±250mV. Oxygen sensor reference voltage may have different values also. For example: for the onboard controllers made by Ford, is 0V, and for the onboard controller of Daimler-Chrysler, is 5V.  
      Onboard controller measures the output voltage signal referenced to the sensor’s signal ground. Oxygen sensor signal ground, depending on its structure, can be connected to a separate connector terminal of the sensor or it can be joined with the sensor housing and in this case, when the sensor is mounted it will automatically connect to the ground of the vehicle by a threaded coupling. In most of the cases the oxygen sensor signal "ground" is connected with a separate wire to the chassis ground. But there are engine management controllers in which this wire is connected to the source of the reference voltage. In such systems, the onboard controller measures the output voltage signal of the oxygen sensor referenced to the source of the reference voltage.
      When the engine is warmed to its operating temperature, the onboard controller by watching the sensor’s output signal assesses the deviation from the stoichiometric mixture (the ideal air/fuel ratio) and decides what to do later. In case of a fully burned fuel mixture, the output voltage signal of the oxygen sensor will be 445¸450mV. But the distance between the exhaust valves and the place where the sensor is installed as well as the significant time for response of its sensitive element, causes a system delay, which doesn’t prevent continuous maintenance of the stoichiometric fuel mixture. Practically, when the engine is in a steady state, fuel mixture deviates from the stoichiometric in range ±2% ~ ±3% with frequency 1 ~ 2 times per second. This process can be assessed very well by observing the output signal waveforms of the oxygen sensor. Transition time of the output voltage should not exceed 120mS from one level to another. Increasing the time can cause sensor aging or poisoning. Sensor poisoning may occur if a lead containing or other components additives to the fuel or to the oil are used. This can happen also if sealing substances are used during an engine repair. Aging of the sensor is due to its continuous operation in aggressive environment at high temperatures.
      Analysis of the oxygen sensor’s output waveforms in different engine modes helps determining sensor or engine management system failures as a whole. The operational resource of the oxygen sensor is 20000km¸80000km. Due to the sensor aging its initial output electrical resistance reduces at a significantly higher temperature of the sensitive element to a value at which the sensor is able to control the reference voltage. Due to the increased electrical resistance, the magnitude of the output voltage signal decreases. Aging oxygen sensor can be easily detected by watching the output waveform when the exhaust flow and temperature are reduced. These are the idle and the low load modes. Practically the old oxygen sensor can still operate in a car, but once the engine load is in idle mode, the output signal magnitude reduces faster until it completely disappears. The output voltage signal is almost steady and its value approaches the reference voltage (300¸600 mV).
      If a significant temperature rise of the sensitive sensor element occurs, the output electrical resistance decreases and the ability to control the reference voltage decreases. This feature of the oxygen sensor allows the diagnostician by increasing the exhaust temperature and flow by rising the engine load or speed, to heat the sensitive element of the sensor to a significantly higher temperature. If in this mode of engine operation the output signal waveform indicates normal values, the oxygen sensor can still provide approximately the specified operational fuel mixture. Moreover, the car owner will not notice an increased fuel consumption and reduction of engine power but idling may be unstable and fluctuation of idle speed may appear.
      Some failures of the oxygen sensor can cause a negative polarity signal occurrence. In case of such a failure, fuel consumption highly increases, engine agility reduces considerably, snap acceleration may cause soot appearance in the exhaust pipe and the spark plugs working surface covers with soot. Failure arises when an internal and sometimes an external leakage of the oxygen sensor is present. The sensitive element of the sensor compares the level of oxygen in the exhaust and in the air. In case of a significant difference between the oxygen level in the air chamber and the exhaust, the sensor generates a voltage ~ 1V. Polarity of this voltage depends on which of the chambers has a decreased level of oxygen. In a properly operating system the level of oxygen changes only by the exhaust side and only downwards. Level of oxygen in the atmospheric chamber is significantly higher than the level in the exhaust and thus the sensor generates a positive polarity voltage of 1V.
      In case of an oxygen sensor depressurization, exhaust gases with low oxygen content enter the atmospheric chamber. In deceleration mode (throttle valve closed, high engine rpm and fuel is being supplied) almost pure atmospheric air is exhausted. In this case the oxygen level in the exhaust system increases dramatically and the oxygen level in the sensor’s atmospheric chamber is significantly lower than that in the exhaust, so the sensor generates a negative polarity voltage of 1V. In this case the onboard controller considers the oxygen sensor correct because after the engine has been started and warmed, the sensor have lead away the reference voltage and lowered it to ~0V. Such output voltage corresponds to a level close to the exhaust oxygen level and to a depressurized atmospheric chamber of the sensor. The onboard controller considers the fuel mixture leaned. Consequently, the onboard controller enriches the fuel mixture. Thus oxygen sensor leaks leads to increased fuel consumption. Many self-test systems could not detect such a failure.

Operation description of the titanium oxide oxygen sensor 
      Output voltage signal range of the titanium oxide sensor is from 10mV¸100mV to 4V¸5V. In such sensor changes in the exhaust gases composition causes reduction of the sensor’s electrical resistance. The electrical resistance is high at low oxygen content in the exhaust gases (enriched fuel mixture) and rapidly reduces with the fuel mixture leaning. Furthermore, the sensor shuts the 5V reference voltage coming from the onboard controller through a resistor with a constant electrical resistance. Output signal of this type sensor reacts much faster to changes in the exhaust oxygen level in comparison to the reaction of the zirconium oxide sensor.

Operation description of the wideband oxygen sensor
      Output signal of the wideband oxygen sensor, unlike the sensors with two levels, provides information not only for the direction of variation of the fuel mixture composition, but to its numerical value. By analyzing the level of the output signal, the onboard controller reads the variation coefficient value of the fuel mixture composition, which in practice is the lambda coefficient. For the Bosch wideband sensors the sensitive element output voltage (black wire referenced to the yellow) varies depending on the exhaust oxygen content and on the electric current polarity flowing in the oxygen pump (red wire referenced to the yellow). The onboard controller generates electric current and supplies the oxygen pump, value and polarity of which provides keeping the sensitive element of the sensor to a set level (450mV).
      If the engine runs with the stoichiometric fuel mixture, the onboard controller would have supplied the red wire with voltage equal to that of the yellow wire and the current flowing through the red conductor and oxygen pump of the sensor would be zero. When engine runs with leaned fuel mixture the onboard computer supplies the red wire with positive voltage referenced to the yellow wire and a positive polarity current starts to flow through the oxygen pump. When engine runs with enriched fuel mixture, the onboard controller changes the both the red wire voltage polarity and current polarity. Currents flowing through the oxygen pump, set by the onboard controller, depend on the value of the deviation of the fuel mixture from the stoichiometric.
         A measurement resistor is connected in the oxygen probe circuit which voltage drop indicates the exhaust gases oxygen level.

Procedure for functionality verification of the lambda sensor

  • Identify the terminals. Sensor may have one, two, three or four terminals depending on the system being tested:
    • Oxygen sensor heater ground (white);
    • Oxygen sensor heater positive power (white);
    • Lambda probe signal (usually black wire);
    • Lambda probe ground (usually gray).
  • Check the oxygen sensor heater, if it is present. Check for heater supply equal to that of the car battery - 12V. If there is no voltage, check the wires to the ignition key relay. Check the oxygen sensor heater connection to the ground.
  • Start the engine and heat it up to its operating temperature.
  • Keep engine speed at 3000 rpm for 30 seconds. This will increase the temperature of the sensor, i.e. it will turn on.
  • Keep engine speed at 2500 rpm. If the engine is idling for a long period of time, oxygen sensor will cool and will turn off.
    Note: this test can’t be performed with a faulty thermostat.
  • Determine the type of the oxygen sensor - zirconium, titanium or wideband.
  • Check the output signal of the oxygen sensor.
    • Zirconium sensor before the catalytic converter.
      NOTE: The digital voltmeter will read an average voltage, for example 450mV. The "slow" oxygen sensor can turn on correctly and still not to be noticed that the voltage is slightly higher. In most case the oscilloscope is the most appropriate instrument for finding faults. It is not appropriate to use a voltmeter or a fault code reader.
      If the sensor is working properly, when engine is in idle mode, steady, close to sinusoidal waveform fluctuations with frequency 1Hz÷5Hz can be seen on the oscilloscope screen. The smallest value of the signal is 0.1V and the maximum value is 0.9V. Average level of fluctuations will be around 0.45V. Duration of the signal edges is not greater than 250ms. Same signal but with a higher frequency should be observed at higher engine speeds.
    • Zirconium sensor after the catalytic converter.
      With a properly working catalytic converter, the oxygen sensor signal will be a straight line at level 0.5V…0.6 V. Output voltage can also be measured with a digital voltmeter. If signal varies and it is close in shape to the signal from the sensor before the catalytic converter, this means the catalytic converter is not working properly.
    • Titanium sensor before the catalytic converter.
      If the sensor is working properly, when engine is in idle mode, signal fluctuations can be seen in the range of 0.2V to 4.5V, and with steeper edges compared to the ones in the case of a zirconium sensor. Digital voltmeter will read an average voltage of around 2V.
    • Wideband oxygen sensor

      Unlike narrow band sensors that communicate to the computer by means of a voltage on a single wire, the wide band sensor uses two wires and signals the computer by means of a current flow. An air/fuel ratio of 14.7 to 1 (by weight), is considered to be the optimum air/fuel ratio. When the ratio is above this value, the current flows in one direction, and when it is below this value it flows in the other. When the air/fuel ratio is exactly 14.7 to 1, the current doesn't flow at all. In order to signal increasing rich or lean conditions, the current flow increases in ratio to how rich or lean the air/fuel ratio is. The two wires are called the current pump wires. The voltages on these current pump wires varies from manufacturer to manufacturer. One of the 2 current pump wires will have a voltage supplied to the sensor by the ECU. The other wire will be a return wire from the sensor to the ECU. Toyotas have 3.0 volts on their reference wire and the 3.3 volts on the current return wire. Note that the 3.3 volts will vary slightly as the current flows, but these changes are very tiny. Likewise, Nissans use 2.7 volts on their reference wire, and the current wire is approximately 3.0 volts. So far, in all of the 4-wire wide band sensors we've seen, the difference between the 2 current pump wires has been a nominal .300 (300 millivolts), that fluctuates slightly based on current flow.
      There is another type of wide band sensor that uses 5 wires, and sometimes 6 wires (rare). In this case there is a 5th wire that gives a voltage representation of the current flow on the current pump wires. When a 5th wire is used in this way, it will usually be called the "signal wire". The 6-wire versions also supply a ground reference for the signal wire. In both of these cases, there is circuitry to convert the current flow on the current pump wires into a voltage. But this type still uses the current pump pair of wires to control the voltage on the 5th wire. 
      • Clip the positive multimeter lead to the signal wire terminal on the sensor. The signal wire terminal is the third one in from the side (middle out of the five terminals). 
      • Clip the negative mulitmeter lead to a grounded point. A grounded point can be the negative battery terminal or the metal surface of the manifold or engine.
      • Turn the engine on and allow the vehicle to idle for one minute. 
      • Monitor the mulitmeter; you should see a reading between 1 and 5 volts. If you are getting no reading; the sensor is faulty and should be replaced.

Oscilloscope measurements

  • Connect the active oscilloscope probe to the signal output of the oxygen sensor and the ground probe - to chassis ground.
  • Repeat all procedures for warming up the engine described above.
  • Output signals of a proper operating zirconium oxide oxygen sensor placed before the catalytic converter, are shown in fig. 2 and fig. 3.

Fig. 2

Fig. 3
Output signal of a properly operating titanium oxide oxygen sensor is shown in fig. 4

Fig. 4
Output signals in case of an leaned or an enriched fuel mixture are shown respectively in fig. 5 and fig. 6.

Fig. 5

Fig. 6

Important addition
Besides all described above, oxygen sensor replacement is mandatory in the following cases:

  • If edge signal duration, observed on the oscilloscope screen, exceeds 350ms when zirconium sensor is present.
  • Pulsation is higher than 0.2V.
  • The maximum value of the output signal is below 0.8V.

      Conclusion of changing the oxygen sensor will be rash if it is taken only on the basis of the fact that its output voltage is too low. This may be due to too lean fuel mixture or due to a leakage of air, and the sensor will not be the reason for this.
      Also if a heated oxygen sensor is present, heater resistance should be checked as well as the presence of 12V supply to the heater.