Oxygen Sensor: Description and Operation

4-Wire Heated Oxygen Sensor (Cut-Away View):

PURPOSE AND LOCATION
The oxygen sensor, installed in the exhaust manifold, monitors exhaust gas oxygen content and produce a voltage output. The PCM uses this voltage value to calculate the percentage of oxygen in the exhaust gas and adjusts the fuel injector pulse width to maintain the correct air/fuel ratio.

CONSTRUCTION AND OPERATION
The sensor is a four wire/electrically heated type. It consists of a zirconia element as the sensor and a heating element to maintain its temperature during times when it might cool below operating temperature (such as decelerating or prolonged idle conditions). The element is a porous tube shape with one end closed, that protrudes into the exhaust. It is impregnated with a Platinum coating on its inside and outside surfaces. The open end of the element is exposed to the atmosphere and its surface is connected to the ECU, while the closed end protrudes into the exhaust stream and its surface is connected to ground. The oxygen sensor element only reacts efficiently with oxygen if the temperature of the element is around 300~C (575~F) or hotter. When there is a difference in the amount of oxygen reacting with the inner and outer surfaces, a voltage potential is generated. The normal operating voltage range is approximately 0.1-1.0V. Voltage increases when the concentration of oxygen in the exhaust is low (rich air/fuel ratio) and decreases when the concentration is high (lean air/fuel ratio).

Oxygen Sensor Output Voltage vs. Air/Fuel Ratio:

The difference in oxygen concentration in the narrow range of optimum air/fuel ratio causes a large voltage change that is easily measured.