What an Oxygen Sensor does
The purpose of the Oxygen sensor is to send a voltage signal to the Powertrain Control Module, or PCM, which indicates exhaust oxygen concentration. This is required so the PCM will adjust fuel injector control to achieve the ideal air/fuel ratio of 14.7:1.
Oxygen is required to combust fuel. If the ratio of 14.7:1 is held, there will be a small but measurable amount of oxygen in the exhaust gas stream. When there is too much fuel, all of the oxygen gets consumed and there is none left. When there is too little fuel, there is a lot of oxygen left in the exhaust stream. The oxygen sensor develops a voltage between 0 and 1 which relates how much oxygen there is left over after combustion.
Here's how it works: the sensor contains zirconium dioxide, a compound capable of generating its own voltage under certain conditions. The compound has one side exposed to the exhaust gases, and the other exposed to outside air. The difference between the outside air and exhaust stream oxygen content is what generates the voltage. A smaller difference, which indicates a lean condition, generates a smaller voltage, under 0.5 volts. A larger difference, such as a rich mixture where there is no oxygen left in the exhaust, generates a higher voltage, over 0.5 volts. The voltage constantly cycles back and forth above and below 0.5 volts, because the PCM is constantly adjusting the fuel delivery to keep up with changing conditions.
One condition is required for the zirconium dioxide sensor element to work properly: high heat. The element must be over 600 F to function properly. To achieve this quickly, and keep it warm, there is a heating element in all modern oxygen sensors. This element is activated by the PCM at startup, and is controlled to keep the temperature of the sensor hot enough to do its job. These sensors typically have four wires, and the PCM can set a variety of diagnostic trouble codes related to oxygen sensor and heater operation.
Monitoring an oxygen sensor can be done using a diagnostic scan tool, or an oscilloscope attached directly to the signal wire coming from the oxygen sensor. Care must be taken. The wrong tool attached to the sensor can damage both the sensor and the PCM, a very expensive mistake.
A word about the air/fuel ratio. An engine can actually run at a wider variety of mixture ratios, and some are even more fuel efficient. The most important reason for maintaining the ideal ratio is catalytic converter operation. This is a device that cleans pollutants out of the exhaust stream, but it must have as close to the ideal ratio as possible to best do its job. The ideal ratio is actually most ideal for converting hydrocarbons, carbon monoxide, and oxides of nitrogen into carbon dioxide and water, in the presence of the elements in the catalytic converter.
