Oxygen Sensor: Description and Operation

Heated Oxygen Sensor Cutaway View:

Oxygen Sensor Element:

OPERATION
The Heated Oxygen Sensor (HO2S) is essentially a small variable battery; it has the ability to produce a low voltage signal that feeds information on engine exhaust oxygen content to the Powertrain Control Module (PCM).

CONSTRUCTION
The HO2S is constructed from a material (zirconia/platinum) that conducts electricity under certain conditions. At operating temperature, 315°C (600°F), the element becomes a semiconductor. A platinum coating on the outer surface of the element stimulates further combustion of the exhaust gases on the H2OS surface which helps keep the element up to the desired temperature. The HO2S has an intercavity which is filled with atmospheric (reference) air. The reference air has approximately 21% oxygen in it. In this electrical circuit this intercavity is the positive (+) terminal. The outer surface of the element is exposed to the exhaust gas stream. It is the negative (-) or ground terminal. The oxygen concentration differences between the reference air and exhaust gases produce small voltages.

VARIABLE HO2S VOLTAGE
A rich exhaust (excessive fuel) has almost no oxygen. When there is a large difference in the amount of oxygen touching the inside and outside surfaces, there is more conduction, and the sensor puts out a voltage signal above 600 mV. With lean exhaust (excessive oxygen) there is about two percent oxygen in the exhaust. This is a smaller difference in oxygen from the outside surfaces which results in less conduction and a voltage signal below 300 mV. The voltages are monitored and used by the PCM to "fine tune" the air/fuel ratio to achieve the ideal mixture desired.

PCM REFERENCE VOLTAGE
The PCM sends a reference signal of 450 mV. The reference signal serves to run the engine when it is in "Open Loop" mode of operation. When the air/fuel ratio is correct the PCM displays 450 mV. When the engine is operating with a rich air/fuel ratio, there is a reduction of free oxygen in the exhaust stream and the oxygen voltage rises above the reference voltage.

VOLTAGE/AIR-FUEL RELATIONSHIP
When the engine is running lean, the voltage drops below the reference voltage due to excess oxygen in the exhaust stream. The HO2S provides the feedback information for the "Closed Loop" operating mode of the fuel delivery system. The H2OS indicates to the PCM what is happening in the exhaust. It does not cause things to happen. It is a type of gauge: Low voltage output = lean mixture = high oxygen content in exhaust; high voltage output = rich mixture = low oxygen content in the exhaust.