Subaru 2002 Impreza Service Manual page 67

SENSORS AND SWITCHES
Fuel Injection (Fuel System)
0
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 avery large difference in the oxygen ion concentration between the inside and
outside of the tube, and the electromotive force generated is large.
0
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).
AtmosDheric air
/-
Zr02
Lean
t - -
Air-fuel ratio
-
Rich
Ceramic heater
1
NF0331
FU-17