Oxygen (O2) Sensor

The exhaust gas or oxygen (O2) sensor is a device used by the vehicle's engine computer to measure the amount of oxygen present in the exhaust stream after combustion has taken place. This information is used by the computer to maintain the ideal air/fuel ratio.
The heart of oxygen sensor is a hollow cone-shaped piece of ceramic material called zirconium dioxide that is platinum coated on the inside and outside. This platinum coating provides the electrodes, as in a battery. During engine operation, exhaust gases are forced to pass over the outside of the sensor element while the inside is exposed to outside air (also referred to as reference gas). When the amount of oxygen in the exhaust differs from the reference gas, a voltage is produced.
A lean air/fuel ratio mixture has higher oxygen content in the exhaust gas. In this condition, the amount of oxygen ions present on both the inner and outer sensor surface are nearly equal, creating a lower output voltage. When the mixture gets too rich, however, there is a high amount of hydrocarbons in the exhaust as well as a low amount of oxygen. This condition causes an unequal amount of oxygen ions present on the exhaust side of the sensor tip compared to the inside, which causes a proportionately higher sensor voltage to be generated. The voltage output of a normal operating oxygen sensor will fluctuate rapidly back and forth between approximately 100mV (0.1V) and 1000mV (1V) at approximately 1200 to 155 RPM with a warm engine. The sensor switch point is the voltage value at which a radical change in output voltage occurs at around the ideal mixture ratio. Voltages above or below the switch point indicate that the air fuel ratio is rich or lean, respectively. The microcomputer tries to keep the sensor voltage at the ideal switch point by controlling fuel input to the engine.