Oxygen sensor or O2 sensor, is one of the most important electronic sensing device in vehicle. They are located at the exhaust system, before and after the catalyst converter (the pre- and post- sensors). And depends on car, some has single bank and some has 2 banks. So you always find 2 to 4 sensors in the entire system. Dont be suprise some older car may even just come with one O2 sensor around the exhaust manifold.
So what does it do? As it's name, O2 sensor measures the amount of oxygen remained in the exhaust gas after engine combustion and provide voltage reading feedback to the ECU on how lean or rich the air-fuel ratio is, so the ECU can determine the driving condition and make adjustments on other eletronic devices such as the fuel injectors for fuel control in order to make the vehicle runs (burns) more efficiently and reduce emission.
Just like other electronic components, O2 sensors tend to wear out after a long period of time of usage. Once it went bad or failed, the ECU will stop receiving feedback or the voltage reading will be out of the normal range and you will see the engine light illuminated on the dash straight away, or you willl find that your car is running rough and comsuming more fuel than before. A vehicle scan-tool will tell you exactly which O2 sensor is playing up and inspection or replacement can be carried out from there.
They are different types of O2 sensors in the
market, ranging from the 2-wire to 5-wire unit but all will have the similar functionality. Always contact your technician if you have ever came across with an O2 sensor problem. The sensor itself could be expensive and you would not want to shorted the new sensor if you are not fimiliar with wiring and the installation.