Subaru LEGACY 2002 Quick Reference Index page 41

B: REAR OXYGEN SENSOR
! 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 ex-
haust gas contains more oxygen. To the contrary, if the fuel ratio is richer than the stoichiometric
ratio, the exhaust gas contains almost no oxygen.
! 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.
! The rear oxygen sensor has a zirconia tube (ceramic) which generates voltage if there is a dif-
ference 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.
! A ceramic heater is employed to improve performance at low temperatures.
(1) Protection tube
(2) Ceramic heater
(3) Zirconia tube
SENSORS AND SWITCHES
FU-21
Fuel Injection (Fuel System)
(4) Gasket
(5) Sensor housing
(6) Harness
B2H3810C