PURPOSE: To prevent deterioration of a sensor by a detected voltage by providing a voltage control means for decreasing the voltage to be impressed to the metal oxide semiconductor of an exhaust gas sensor in a region where the resistance value of the sensor drops extremely.
CONSTITUTION: A load resistance R1, resistances R2, R3 for setting the detection levels of an air-fuel ratio, and reference resistance R4, R5 for discriminating whether the interruption of a power supply is needed or not are connected to the exhaust gas sensor 6 consisting of the n-type metal oxide semiconductor. The intermediate voltage of a rich level and lean level is taken out by the resistances R2, R3 and whether the air-fuel ratio is rich or lean is discriminated. The output corresponding to the rich level is taken out by the resistances R4, R5 and whether the sensor output is above said level or not is discriminated. Whether the resistance value of the sensor 6 drops down to the value below the resistance value determined by the reference voltage or not is detected in such a manner and the drop of the resistance value of the sensor 6 below the above-mentioned value is adopted as the condition for cutting the detection voltage.
OKINAGA KAZUO
YOSHIDA YUTAKA
FIGARO ENG
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