In order to determine whether the air-fuel ratio (AFR) is rich or lean, the ECU (engine control unit) in a vehicle with electronic fuel delivery control (either carbureted with electronic mixture control, or with electronic fuel injection/EFI) monitors the voltage output of an oxygen sensor (sometimes referred to as an O2 sensor) and adjusts fuel delivery accordingly in order to closely approach a stoichiometric ratio. Automotive oxygen sensors come in two basic types: The zirconium type and the titanium type. By far the cheaper and more common is the zirconium model. There are also linear air-fuel ratio sensors, which are typically also zirconium-based but which can provide feedback of air-fuel ratios from 12:1 all the way up to 22:1.
The zirconium sensor consists of a zirconium dioxide element with a thin platinum coating applied by vapor deposition. When the element heats up to about 600°F (or about 315°C), negative ions of oxygen are attracted to the platinum coating, creating an ionic imbalance that causes electrons to move through the sensor element and induce a voltage. The more oxygen found in the exhaust stream, the higher the voltage, so a high voltage indicates a lean condition in which the vehicle is getting a lot of air and little fuel. Conversely, a low voltage means there is little oxygen in the exhaust, meaning that the air-fuel ratio (AFR) is rich. Stoichiometric ratios of air and fuel should result in the sensor putting out 450mV, and normal operating voltages are from 100mV to about 850mV because the ECU normally operates in what we call closed loop mode where it is constantly fluctuating between rich and lean AFRs in order to most closely approach a stoichiometric ratio.
The linear AFR sensor which is found on some of the most modern vehicles such as the Honda Insight is basically a pair of zirconium sensors and a control element. The control element allows control of exhaust gas flow through the element, and while one sensor is in the exhaust gas stream, the other provides a reference, atmospheric signal. The sensor is heated to ensure operation. O2 sensor output varies significantly with temperature, but this sensor's output remains fairly constant through the use of the heater and is balanced using the atmospheric reference signal. These sensors can usually be identified by their five wires; Heater power and ground, sensor element and control element positive, and a shared ground for the sensor and control elements. LAF sensors produce either a positive or negative voltage, with 1.5 volts being the average operating voltage, and positive voltage indicating a lean mixture.
Titanium sensors are resistive and require a reference voltage, either 5 volts (Jeep) or 12 volts (Nissan) and return the proper voltage to indicate rich or lean behavior.
Your average O2 sensor is from three to five inches long. The first inch or so (probably around 2cm) is the electrode, which is covered with a vented guard to protect it from impacts which could remove the platinum coating. The next three eighths to half an inch (or roughly 1cm) is the thread. This is followed by wrench flats used to turn the sensor so that you can screw it into the socket, and then the body of the sensor from which one to four wires extend.
O2 sensors can be removed or installed with a special socket called a sensor socket. It comes in a couple of different sizes, as there are about two sizes of O2 sensor commonly found. They also can commonly remove some of the larger coolant temperature sensors. They consist of a deep socket with a notch cut down one side to accommodate the sensor wire(s), usually stainless steel wire which is permanently attached to the sensor, as the heat of the exhaust system might be a problem for other types of wiring.
The repeated expansion and contraction of the exhaust manifold (or other O2 sensor location) due to heating and cooling, combined with the effects of oxidation and fouling due to soot can make it very difficult to remove an O2 sensor, which may tempt you to put anti-seize compound on the threads. This is a bad idea except when the compound is either included with the sensor or already applied to its threads. Even some fuel additives may be able to foul an O2 sensor, damaging its platinum coating and causing it to return consistently low voltage signals, and anti-seize compounds will do likewise, especially those which contain silicone. When anti-seize compound is used, care should be taken to get it only on the threads.
As mentioned previously, O2 sensors can have from one to four wires. The most common is the one wire, which is heated by exhaust gases and uses the engine as the ground for the return signal. The desire to use a separate ground led to 2-wire sensors. Three and four wire sensors have a heating element integrated into the sensor; three wire sensors have a heater power wire, a heater ground wire, and a signal wire, with the signal return running through the chassis ground. Four wire sensors have a signal return wire. While heated O2 sensors are most common when they are somewhere other than inside the exhaust manifold, for example after the catalytic converter, they may be installed anywhere. They are most common on OBD-II vehicles, but may be found on older cars as well, especially Japanese imports.
While vehicles used to have only one or two O2 sensors, modern cars may have as many as four or five. Typically, older (but still computer-controlled) vehicles have one O2 sensor per exhaust system, so if they have a dual exhaust they have two of them, typically located in the exhaust manifolds. Vehicles with a single exhaust system and a V-shaped motor might have one in each manifold, or one heated sensor where the two manifolds come together. Since 1996 and the inception of OBD-II, however, it has been necessary to install a heated O2 sensor behind the catalytic converter to monitor its efficiency. If the output from this sensor does not change significantly during operation then the catalyst is considered to be working properly. In addition, O2 sensors can be a helpful part of the fuel system monitors (see OBD-II) and will help to detect fuel injector drips, leaks and other problems. It is not unusual for a modern V8 to have one sensor per manifold, one more before the catalyst, and one after, or one in each manifold, and one after each of two catalysts.