See Figures 1 and 2
The oxygen sensor is essentially a small variable battery; it has the ability to produce a low voltage signal that feeds information on engine exhaust oxygen content to the control module.
The sensor is constructed from a zirconia/platinum electrolytic element. Zirconia is an electrolyte that conducts electricity under certain chemical conditions. The element is made up of a ceramic material which acts as an insulator when cold. At operating temperatures of approximately 600°F (315°C), the element becomes a semiconductor. A platinum coating on the outer surface of the element stimulates further combustion of the exhaust gases right at the surface and this helps to keep the element up to the desired temperature.
The oxygen sensor has an inner cavity which is filled with reference (atmospheric) air. The atmosphere has approximately 21 percent oxygen in it. In the circuit, this inner cavity is the positive terminal, while the outer surface (exposed to the exhaust stream) is the negative or ground terminal.
Due to the element's electrolytic properties, oxygen concentration differences between the reference air and exhaust gases produce small voltages. A rich exhaust (excess fuel) has almost no oxygen. So when there is a large difference in the amount of oxygen touching the inside and outside surfaces, more conduction occurs and the sensor puts out a voltage signal above 0.6 V (600 mV). The signal may vary as high as 0.9 V (999 mV).
With a lean exhaust (excessive oxygen), there is about 2 percent oxygen in the gases. The smaller difference in oxygen content causes less conduction and the sensor produces a smaller voltage somewhere below 0.3 V (300 mV). The signal could drop as low as 0.1 V (100 mV). Commonly, values outside this range will cause a trouble code to set for most control systems.
Some late model vehicles are equipped with a heated oxygen sensor. This sensor has 3 wires in its pigtail instead of 1. The heated sensor works much in the same way as the standard oxygen sensor. The difference comes in the 2 other wires. On 1 wire, battery voltage is applied to a heating element located inside the sensor, while the other wire completes the heating elements circuit to ground. The heating element helps the sensor reach and maintain operating temperature faster, thereby allowing the fuel injection system to reach closed loop operation sooner.
Removal and installation of the oxygen sensor is covered in Engine & Engine Overhaul of this guide. Please refer there for further information.