The oxygen sensor in a car is now an integral part of the exhaust system. Although it has been around for many years--since the implementation of On-Board-Diagnostics II in 1996--the amount of oxygen sensors per car has doubled. Finding them for replacement is now much trickier.
The oxygen sensors are located directly in the exhaust system, usually by portholes that the sensor screws into on the exhaust pipes--and in some cases, on the exhaust manifold. The exterior portion of the sensor provides a wire and plug that plugs the harness into the ECU and transmits the voltage for monitoring.
Designed in the 1960s by a Robert Bosch, the oxygen sensor in a car made its first introduction in the late 1970s on a Volvo that also introduced the 3-way-catalyst system. By 1981, to reduce emissions, all vehicles were being manufactured with catalytic converters.
The function of the oxygen sensor (also known as the Lambda sensor) is to monitor the amount of fuel-to-air mix in the combustible engine of a vehicle. The sensor transmits voltage and the engine management computer (ECU) monitors the ratio. There is an ideal ratio--when a sensor fails or there is an internal problem with the engine, more pollutants emit from the exhaust.
The older sensors were "unheated" sensors. Since they do not transmit voltage until their internal temperature reaches 600 degrees Fahrenheit or more, this allows a period of time in between the ECU being able to monitor the sensor. "Heated" sensors are common today, have extra wires for the heating element and assist in heating the sensor more rapidly.
If replacing an oxygen sensor, a direct-fit replacement is far superior to the "universal" sensor which would require cutting, identifying and splicing the wires of the sensor. The direct-fit sensor provides the necessary wires and plug to fit the plug of the wire harness easily.
