Legacy Outback Ltd Sedan F4-2.5L SOHC (2000)
Oxygen Sensor: Description and Operation
Rear Oxygen Sensor
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The rear oxygen sensor is used to sense oxygen concentration in the exhaust gas. If the fuel ratio is leaner than the stoichiometric ratio in the
mixture (i.e.. excessive amount of air), the exhaust gas contains more oxygen. To the contrary, if the fuel ratio is richer than the stoichiometric
ratio, the exhaust gas contains hardly any oxygen.
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Therefore, examination of the oxygen concentration in exhaust gas makes it possible to show whether the air/fuel ratio is leaner or richer than the
stoichiometric ratio.
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The rear oxygen sensor has a zirconia tube (ceramic) which generates voltage if there is a difference in oxygen concentration between the inside
and outside of the tube. Platinum is coated on the inside and outside of the zirconia tube for the purpose of catalysis and electrode provision. The
sensor housing is grounded to the exhaust pipe, and the inside is connected to the ECM through the harness.
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A ceramic heater is employed to improve performance at low temperature.
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When rich air-fuel mixture is burnt in the cylinder, the oxygen in the exhaust gases reacts almost completely through the catalytic action of the
platinum coating on the surface of the zirconia tube. This results is a very large difference in the oxygen concentration between the inside and
outside, and the electromotive force generated is large.
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When a lean air-fuel mixture is burnt in the cylinder, oxygen remains in the exhaust gases even after the catalytic action, and this results in a small
difference in the oxygen concentration. The electromotive force is very small.
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The difference in oxygen concentration changes greatly in the vicinity of the optimum air-fuel ratio, and hence the change in the electromotive
force is also large. By inputting this information into the ECM, the air-fuel ratio of the supplied mixture can be determined easily. The oxygen
sensor does not generate much electromotive force when the temperature is low. The characteristics of the electromotive force stabilize at