Today's computerised engine control systems rely on inputs from a variety of sensors to regulate engine performance, emissions and other important functions. The sensors must provide accurate information otherwise drive-ability problems, increased fuel consumption and emission failures can result.
One of the key sensors in this system is the oxygen sensor. It's often referred to as the "O2" sensor because O2 is the chemical formula for oxygen (oxygen atoms always travel in pairs, never alone).
The first O2 sensor was introduced in 1976 on a Volvo 240. Many vehicles are now equipped with multiple O2 sensors.
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the
exhaust as the exhaust exits the engine. Monitoring oxygen levels in the exhaust is a way of gauging the fuel mixture. It tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors can affect the relative richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. There are other sensors to monitor these factors, too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Consequently, any problems with the O2 sensor can throw the whole system out of sync.
"The cost of replacing an O2 Sensor can vary due to the location, accessibility and ease of removal. A special tool is often required. The Sensor itself is made using a number of precious metals and costs anything from £30 to £150, most cars have two sensors but if you have a "v" engine then your car will have 4 O2 Sensors"
The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.
The outside of the bulb is exposed to the hot
gases in the exhaust while the inside of the bulb
is vented internally through the sensor body to
the outside atmosphere. The difference in oxygen
levels between the exhaust and outside air within
the sensor causes voltage to flow through the ceramic
bulb. The greater the difference, the higher the voltage reading.
An oxygen sensor will typically generate up to about 0.9 volts
when the fuel mixture is rich and there is little unburned oxygen
in the exhaust. When the mixture is lean, the sensor's output
voltage will drop down to about 0.1 volts. When the air/fuel
mixture is balanced or at the equilibrium point of about 14.7 to 1,
the sensor will read around 0.45 volts.
When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system.
Starting with a few vehicles in 1994 and 1995, and all 1996 and newer vehicles, the number of oxygen sensors per engine has doubled. A second oxygen sensor is now used downstream of the catalytic converter to monitor the converter's operating efficiency. On V6 or V8 engines with dual exhausts, this means up to four O2 sensors (one for each cylinder bank and one after each converter) may be used.
The OBDII system is designed to monitor the emissions performance of the engine. This includes keeping an eye on anything that might cause emissions to increase. The OBDII system compares the oxygen level readings of the O2 sensors before and after the converter to see if the converter is reducing the pollutants in the exhaust. If it sees little or no change in oxygen level readings, it means the converter is not working properly. This will cause the Malfunction Indicator Lamp (MIL) to come on.