Heated Oxygen Sensors
Oxygen sensors are located in the exhaust manifold or exhaust pipe of the engine's exhaust system. The sensor measures the oxygen content in the exhaust stream. It sends an analog voltage signal to the PCM that varies between 0 and 1 volt. The PCM uses this signal to control and adjust the engine's air-fuel ratio.
This sensor produces a reliable voltage signal after it reaches the temperature of 600° F. This is due to the design and materials used to create the sensor. The most common type, the zirconia sensing element is a conductor of voltage when it's hot and a resistor when it's cold. Oxygen passing through the element affects the voltage signal. It produces a high voltage signal (.90) when the engine is running rich and a low signal (.10) when the engine is running lean.
The sensor can have one to four wires. A one-wire or two-wire sensor does not contain a heater circuit. This sensor does not send a voltage signal until it reaches operating temperature. Three-wire and four-wire O2 sensors have a heater that brings the sensor to operating temperature and producing a voltage signal sooner. The only difference between the two is that the three-wire sensor shares a common ground between the heater and sending circuits. Because it reaches operating temperature sooner, a heated oxygen sensor is more efficient than the unheated sensor.