An Oxygen Sensor (also called an O2 Sensor) comes in various flavors; Wideband, Analog, and Heated.

First of all, an O2 sensor is a device typically located in the Y-pipe of the exhaust system of a vehicle, this round device takes an Fuel/Air mixture reading of the exhaust fumes by determining the ammount of Oxygen present.

On most newer cars equipped with OnBoard Diagnostics there is a second O2 sensor located just behind the Catalytic Converter which determines how well the converter is operating. Either of these sensors, if they fail, or detect an abnormal problem will trip the Services Engine Soon light.

The primary sensor makes a comparison between the atmosphere Oxygen and the Oxygen levels in the exhaust stream. This is called the Air/Fuel ratio.

Onto the types of sensors:

All sensors report back a voltage to the computer, typically with one to three wires connected to the sensor. Analog sensors are the most common, they are complete chemical power generators and produce their own voltage which is fed back to the computer via one single wire. These types of Sensors are slow to respond to sudden A/F ratio changes. These sensors report in millivolts and are not powerful enough to operate any analog in-car gauges.

The second type, and more common, is the Heated Analog sensor. When the car first starts, and the internal engine tempature is below 600F, the car refuses to accept any readback from this sensor for a period of time, and when other conditions are met. A Heated Sensor alieviates this problem, heating the sensor as soon as the engine is started, providing a much faster readback to the computer. In the long run, this saves on fuel. This sensor, like the analog, reports in millivolts.

The third type, and least common sensor, is the Wideband sensor. The special thing about this, aside from price, is it's ability to report to the computer more accurately, much faster, and in volts, between 0 and 1 volts, instead of the much smaller millivolt ranges.

Performance people usually purchase the Wideband O2 sensor for it's ability to accurately report the A/F ratio. However, these sensors do not come cheap.

These sensors fail about every 60 to 85k miles, and can fail sooner under excessive stress, such as an overly rich condition, or using leaded fuel. When they go, they can be a pain to change. The exhaust pipe in that area and the sensor together tend to weld together in rust. Since the exhaust Y-pipe is an aluminum alloy, and the threads on the O2 sensor are aluminum, you can very eaisly strip them, break them, or worse, lose the O2 sensor parts in the exhaust system. Take care when changing your O2 sensor. It's also probably best to tackle the problem from underneeth the vehicle, with the engine slightly warm to the touch, since above it, makes it harder to reach.