Oxygen Sensor: Description and Operation

Heated Oxygen Sensor (HO2S):

DESCRIPTION
The Heated Oxygen Sensor (H02S) 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 control module.

CONSTRUCTION
The HO2S is constructed from a zirconia/platinum electrolytic element. Zirconia is an electrolyte that conducts electricity under certain chemical conditions. The element is made of a ceramic material and is an insulator when cold. 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 right at the surface and this helps keep the element up to the desired temperature. Also a heating element is incorporated into the sensor to aid in warm up to the operating temperature and also to maintain that temperature. The HO2S has an inner cavity which is supplied with atmospheric (reference) air from the sensor wires. The atmosphere has approximately 21% oxygen in it. In the electrical circuit this inner cavity is the positive (+) terminal. The outer surface of the element is exposed to the exhaust gas stream and is the negative (-) or ground terminal.

If the heated oxygen sensor pigtail wiring connector or terminal is damaged, the entire oxygen sensor assembly must be replaced. Do NOT attempt to repair the wiring, connector or terminals. In order for the sensor to function properly, it must have provided to it a clean air reference. This clean air reference is obtained by way of the oxygen sensor signal and heater wires. Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference and degraded oxygen sensor performance.

OPERATION
Due to the electrolytic properties of the element, the oxygen concentration differences between the reference air and exhaust gases produce small voltages. 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 0.6 volt (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 0.3 volt (300 mV). The voltages are monitored and used by the control module to "fine tune" the air/fuel ratio to achieve the ideal mixture desired.

The control module puts out a reference signal of 0.45 volt (450 mV). The reference signal serves two purposes. The first is to run the engine when it is in "Open Loop" mode of operation. When the air/fuel ratio is correct the control module "senses" 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 Sensor (O2S) voltage rises above the reference voltage.

When the engine is running lean the voltage drops below the reference voltage due to excess oxygen in the exhaust stream. The oxygen sensor provides the feedback information for the "Closed Loop" operating mode of the fuel delivery system. The 02S indicates to the control module what is happening in the exhaust. It does not cause things to happen. It is a type of gage: Low voltage output = lean mixture = high oxygen content in exhaust; high voltage output = rich mixture = low oxygen content in the exhaust.

An open HO2S should set a Diagnostic Trouble Code (DTC) 13. A constant low voltage in the sensor circuit should set a DTC 44. A constant high voltage in the circuit should set a DTC 45. DTC 44 and DTC 45 could also be set as a result of fuel system problems.
The integral heater in the sensor will not set a DTC but is diagnosed in DTC 13.
For conditions that can cause a lean system refer to DTC Chart 44.
For conditions that can cause a rich system refer to DTC Chart 45.
All the above DTC charts can be found at Procedures / Diagnostic Charts . Testing and Inspection