Subaru LEGACY 2002 Quick Reference Index page 202

Fuel Injection (Fuel System)
! 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 zir-
conia tube. This results in a very large difference in the oxygen ion concentration between the in-
side and outside of the tube, and the electromotive force generated is large.
! 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.
! 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 characteris-
tics of the sensor stabilize at a temperature of approximately 300 to 400°C (572 to 752°F).
SENSORS AND SWITCHES
FU-22
To be judged
as lean
Lean Air-fuel ratio Rich
G2H0038B