Impreza 2.5 RS Sedan AWD F4-2.5L SOHC (2002)
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 air-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 almost no oxygen.
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Detecting the oxygen concentration in exhaust gas using the oxygen sensor makes it possible to determine whether the air-fuel ratio is leaner or
richer than the stoichiometry.
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The rear oxygen sensor has a zirconia tube (ceramic) which generates voltage if there is a difference in oxygen ion concentration between the
inside and outside of the tube. Platinum is coated on the inside and outside of the zirconia tube as a catalysis and electrode material. 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 temperatures.
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When rich air-fuel mixture is burnt in the cylinder, the oxygen in the exhaust gases is almost completely used in the catalytic reaction by the
platinum coating on the external surface of the zirconia tube. This results in a very large difference in the oxygen ion concentration between the
inside and outside of the tube, and the electromotive force generated is large.
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When a lean air-fuel mixture is burnt in the cylinder, relatively large amount of oxygen remains in the exhaust gases even after the catalytic action,
and this results in a small difference in the oxygen ion concentration between the tube's internal and external surfaces. The electromotive force in
this case is very small.
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The difference in oxygen concentration changes drastically in the vicinity of the stoichiometric air-fuel ratio, and hence the change in the
electromotive force is also large. By using this information, the ECM can determine the air-fuel ratio of the supplied mixture easily. The rear
oxygen sensor does not generate much electromotive force when the temperature is low. The output characteristics of the sensor stabilize at a
temperature of approximately 300 to 400 °C (572 to 752 °F).
