OXYGEN SENSORS

Why Are Oxygen Sensors Fitted?

Everyone agrees that the amount of pollution produced by motor vehicles should be reduced. In order to encourage this, governments have introduced tougher and tougher exhaust gas emission legislation.

The most popular method used by vehicle manufacturers is the three-way catalyst. This device converts the main pollutants in the exhaust gas to less harmful gasses. However, the three-way catalyst only works efficiently if the air-fuel ratio can be kept within very tight limits. This is where the oxygen sensor fits into the picture.

The oxygen sensor is typically situated in the exhaust pipe just before the three-way catalyst. The central element of the oxygen sensor is exposed to the exhaust gas.

The oxygen sensor continually detects the oxygen content in the exhaust gas. Its output signal is connected to the ECU and changes to indicate a rich or lean fuel mixture. Through its output signal, the sensor "informs" the ECU if the vehicle is running rich or lean. The ECU uses the information supplied to it by the sensor to decide whether it needs to strengthen or weaken the fuel mixture to achieve optimum air-fuel ratio. This is referred to as closed loop control because the output (oxygen content in the exhaust gas) is measured and fed back to the controller (ECU) which can then correctly control the fuel mixing system. By ensuring that the mixture strength is always correct both combustion and catalyst efficiency are optimised.


How Does The Oxygen Sensor Work?

NGK manufacture two general types of oxygen sensors. The first and more common type uses a zirconia ceramic sensing element and the second uses a titania element. The working principle of the two types of sensors is described in this section.

Zirconia Sensors: An important property of the Zirconia element is that it can conduct oxygen ions above a temperature of about 350 deg.C. When the sensor is fitted, the outside of the Zirconia element is exposed to the exhaust gas and the inside is in contact with air. Both sides of the element are coated with a thin layer of platinum that act as electrodes and carry the sensors' signal (voltage) from the zirconia element to the lead wire. At operating temperature, oxygen ions are able to pass through the element and deposit a charge on the platinum electrode thus generating the voltage signal.

A high signal voltage is generated across the electrodes when different levels of oxygen are present on the two sides of the element. Due to the properties of the Zirconia element there is a large change in sensor voltage when the air-fuel ratio (AFR) is 14.7.

If the AFR is low (rich fuel mixture) the sensor output voltage will be high because the electrode exposed to the exhaust gas is in contact which much lower amounts of oxygen than the one exposed to air. Conversely, if the AFR is greater than 14.7 (lean fuel mixture), the signal voltage will be low.

The ECU uses the voltage produced by the sensor to instruct the fuel mixing system to strengthen or weaken the mixture. A low voltage informs ECU that the fuel mixture is lean and a high voltage that the mixture is rich. The ECU can then take appropriate action to achieve optimum AFR.

The sensor only produces a voltage when the element is above approximately 350 deg.C and it takes the exhaust gas a little while to heat the element up to this temperature after the engine has been switched on. In order to reduce the time it takes for the sensor to reach working temperature, most sensors today are fitted with an internal ceramic heater. These sensors have 3 or 4 lead wires. In the 3 - wire type, two of the wires are required to supply the heater, one wire is used to carry the sensor signal and signal ground is achieved through the manifold. The forth wire in 4 - wire types is used to carry the signal ground (hence the term isolated ground). 1 and 2 wire sensors do not have a heater.

Titania Sensors: The Titania element in these sensors does not produce a voltage like the Zirconia element. The property of the Titania element which allows for the detection of oxygen in the exhaust gas is its electrical resistance. The electrical resistance of the Titania element changes according to the concentration of oxygen in the exhaust gas. There is a big change in the resistance of the element when the fuel-ratio is 14.7. When a voltage is applied to the element in a voltage divider circuit, the output voltage changes with the resistance thus forming the voltage signal required to be processed by the ECU. As with the Zirconia sensor a low output voltage indicates a lean mixture and a high output voltage indicates a rich mixture. These voltages are used by the ECU for closed loop control.

As Titania sensors do not need air on one side of the element, they can be made smaller and are completely submersible. Due to their different properties Titania and Zirconia sensors should not be interchanged under any circumstance.

How To Test Oxygen Sensors?

For the ECU to control the AFR and keep it within tight limits, the oxygen sensor must be working properly. Failed or worn out oxygen sensors cause problems such as poor fuel economy, failed emission tests, failure of the catalytic converter and poor driveability.

Therefore it is important that you can read the signs of a failed or worn oxygen sensor and have the ability to check their performance.

ON CAR TEST: Before you can test the operation of the sensor, you will need an oscilloscope. You should first check that the basic engine set up is to the manufacturers specification, then thoroughly warm up the engine - remember that the sensor will only function once it has reached its operating temperature.

Using an appropriate connecting device, connect the sensor output to your oscilloscope; do not disconnect the sensor from the ECU. Run the engine at approximately 2000 rpm. A properly functioning oxygen sensor will show a rapidly fluctuating output voltage between approximately 0.1 and 1.0 volts. The time taken for the voltage to change from 0.1 V to 1.0 V (referred to as the lean to rich response time) should be about 300 milliseconds. A similar time should be measured when the voltage changes from 1.0 V to 0.1 V (rich to lean response time).

If the sensor output is constant or the response time is too slow the sensor should be changed. It is a god idea to check the oxygen sensor function at every tune up and before submitting cars for emission tests. A slow sensor will effect fuel economy. A new sensor will pay for itself by cutting fuel bills.

Sensor Type Description

Single Wire - One wire carries the sensor signal while ground return is achieved through the vehicle body. Also referred to as an EGO Sensor. (Exhaust Gas Oxygen Sensor).

Two Wires - One wire is the sensor signal and the other is the sensor signal ground. Also referred to as an ISO-EGO Sensor (Isolated Exhaust Gas Oxygen Sensor). The signal ground wire in the NTK two wire (and four wire) oxygen sensor is completely isolated from the vehicle ground. Both sensor signal wires are directly connected to one of the platinum electrodes on the ceramic element. The sensor output is immune to ground loop voltages and also to large resistances in the vehicle ground return, caused by corroded connections. No NTK two wire sensor is case grounded. Case grounded sensors have the ground wire physically attached to the sensor body. It is not recommended to replace isolated ground sensors with case ground types.

Three Wires - One wire carries the signal and two wires are used to supply a voltage to the internal heater. Signal ground return is achieved through the vehicle body. Also referred to as a HEGO Sensor (Heated Exhaust Gas Oxygen Sensor).

Four Wires - One wire is the sensor signal, one is the isolated sensor signal ground and the remaining two are used to supply a voltage to the internal heater. Also referred to as an ISO - HEGO Sensor (Isolated Heated Exhaust Gas Oxygen Sensor).

Visual Inspection





Visual inspection by itself is not usually sufficient to determine if an oxygen sensor is functioning correctly however, the lead wire and connector should be checked for damage. Any damage will interfere with the sensor signal. The sensor body should be checked for dents, which are a sign of mechanical shock that can crack the sensor element. Also, the appearance of the sensor's protection tube can give an indication of possible problems. Below are a few examples of damaged sensors and an indication of what might have been the possible cause. The cause should be rectified and the sensor changed to avoid further problems including damage to the catalytic converter.

	Excessive White or Grey Deposits Excessive deposits such as these indicates that fuel additives are being used or the engine is burning oil. Certain components in the fuel additives and oil can contaminate the sensor element.
	Excessive Carbon or Oil Deposits Excessive carbon and oil deposits can damage the sensor. Carbon deposits can clog the sensor and effect its response time. The cause can be a rich mixture, exhaust air leaks or high oil consumption due to worn piston rings or valve seals.
	Shiny Deposits Shiny Deposits indicate the presents of lead. Lead attacks the platinum on the sensor and in the catalyst. The sensor should be changed and only unleaded fuel used.