Subaru 2002 Impreza Service Manual page 33

SENSORS AND SWITCHES
Fuel Injection (Fuel System)
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When rich air-fuel mixture is burnt in the cylinder, the oxygen in the exhaust gases is almost com-
pletely 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 oxy-
gen 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 informa-
tion, the ECM can determine the air-fuel ratio of the supplied mixture easily. The front oxygen sen-
sor does not generate much electromotive force when the temperature is low. The output charac-
teristics of the sensor stabilize at a temperature of approximately 700°C (1,292"F).
Atmospheric air
/-
Zro2
Lean
-
Air-fuel ratio
-
Rich
Ceramic heater
- . J
NF0331
FU-17