Oxygen Sensor: Description and Operation

Exhaust Oxygen Sensor (02):

PURPOSE
The oxygen sensor (O2S) measures the oxygen content of the exhaust. The ECM monitors the sensor to maintain air/fuel mixture of 14.7 to 1.

CONSTRUCTION
The O2S 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 semi-conductor. 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. The O2S has an outer cavity which is filled with atmospheric (reference) air. 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. It is the negative (+) or ground terminal.


OPERATION
The O2S 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 ECM. The O2S emits a low voltage signal (under 0.2 volts) when fuel mixture is lean (excessive oxygen) and a higher voltage (up to 1 volt) when mixture is rich (almost no oxygen). 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 O2S puts out a voltage signal above 0.6 volt. 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. The voltages are monitored and used by the control module to "fine tune" the air/fuel ratio to achieve the ideal mixture desired.

Voltage signals are not produced by the O2S until exhaust temperature reaches 600° F.

The ECM supplies a reference signal voltage of about .45 volt between the sensor and ground. (If measured with a 10 megaohm digital voltmeter may read as low as .32 volt). The reference signal serves two purposes:

- To run the engine when it is in "Open Loop" mode of operation. When the air/fuel ratio is correct the ECM "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 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.

- To provide feedback information for the "Closed Loop" operating mode mode of the fuel delivery system. The O2S varies the voltage within a range of about 1 volt if the exhaust is rich, down to about .10 volt if the exhaust is lean.