Subaru 2002 Impreza Service Manual page 68

SENSORS AND SWITCHES
Fuel lniection (Fuel System)
B: 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 azirconia tube (ceramic) which generates voltage if there is a differ-
ence 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 har-
ness.
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A ceramic heater is employed to improve performance at low temperatures
(1) Protection tube
(2) Gasket
(3) Sensor housing
NF0390
(4) Ceramic heater
(5) Zirconia tube
FU-18