Oxygen Sensor: Description and Operation

Comp. Loc:

Heated Oxygen Sensor Output Characteristics:

HEATED OXYGEN SENSOR

DESCRIPTION
The Heated Oxygen Sensor (HO2S) is located in the exhaust manifold and detects the concentration of oxygen in the exhaust gases. It consists of the zirconia element (with thin platinum surface coating) which generates voltage, a lead wire which draws out the voltage, and cover and housing which protects the zirconia element from damage. The HO2S also incorporates a heater element. Refer to Chart C-1B Heated Oxygen Sensor Check to verify that HO2S heater is working properly.

The zirconia element, by its property, generates the electromotive force when a difference in oxygen concentration exists between its faces. As its temperature rises, the change of the electromotive force is amplified by catalytic reaction of the platinum. The HO2S makes use of this property. As atmosphere is introduced into the HO2S, the inside of the zirconia element is exposed to the atmosphere and outside to exhaust gases. Thus, the difference in concentration between the inside and the outside of the zirconia element varies with the oxygen concentration in the exhaust gases.

OPERATION
A large oxygen concentration difference results in about 1 volt and a small difference results in about 0.01 volt. To put it in other words, if the amount of oxygen in the exhaust gases is less (air-fuel mixture is richer than the stoichiometric mixture), about 1 volt is generated. If more (air-fuel mixture is leaner than stoichiometric mixture), almost none is generated (Refer to chart in accompanying image). In this way, the HO2S detects whether the oxygen concentration is high or low (or the mixture is leaner or richer than the stoichiometric mixture).