The oxygen sensor is fitted into the exhaust system. There are two common types; the most common of these is the ZIRCONIUM type. This produces a small voltage nominally 0.0 to 1.0 volt. The oxygen sensor has to reach at about 300 degrees centigrade before it will start to work. To increase the speed of warm up early sensors were positioned as close to the engine as possible. Most O2 sensors now used have the addition of a heater built into them; this allows the sensor to be positioned further away from the engine. The voltage generated by the oxygen sensor is dependent on the amount of oxygen present in the exhaust, zero oxygen giving a high voltage and excess oxygen giving a low voltage. By changing the amount of oxygen passing the sensor the cars computer can control the fuelling required for the engine to run at its optimum and the catalytic converter to operate at its most efficient.

To understand how this can be used to control the amount of fuel injected into the engine a little knowledge of the combustion process is needed. The combustion process requires oxygen for it to burn the fuel injected into the engine. If it were possible to create perfect combustion in the engine, all the oxygen entering the engine would be used up in the burning process with no excess fuel (hydrocarbons) left over. Due to cylinder inefficiency this is not possible to achieve, so, we may have a situation where the exhaust contains an excess amount of oxygen (A lean mixture) or zero oxygen where all the oxygen is used up in the burning process. This would leave an excess of fuel (Hydrocarbons) going out with the exhaust.

To stop this from happening the car's computer takes control by altering the fuelling and monitoring the changes in the voltage from the oxygen sensor. The fuel is increased, reducing the amount of oxygen passing the sensor causing the sensor voltage to increase. When the voltage monitored by the cars computer passes a fixed parameter in the computers program (usually 0.45v) the computer will decrease the amount of fuel being injected and this will increase the amount of oxygen passing the oxygen sensor. This increase and decrease in the fuelling is taking place once every second and will not be noticeable to the driver of the car. By using this method the fuelling can be controlled at the ideal of 14.7parts of air to 1 part of fuel.

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