PURPOSE: To diagnose the abnormality of a sensor by a method wherein the current allowed to flow to an electrochemical pump cell is controlled so as to maintain the voltage of an electrochemical sensor cell at a predetermined objective value and an air/fuel ratio sensor is judged to be abnormal when the voltage varies from the objective value by specific value or more.
CONSTITUTION: After a predetermined time is elapsed from the point of time when an ignition Ig is turned ON, the supply of a current to a pump cell 34 is started and an air/fuel ratio sensor 30 is operated to detect the concn. of oxygen in exhaust gas. Subsequently, the output voltage from a voltage detection circuit 5 is monitored by a comparing and judging circuit 7 to calculate the potential difference ΔVS with reference voltage VREF 1. Next, this variation quantity is compared with reference voltage VREF 2. When the voltage of a sensor cell 32 is varied by 0.1V or more from an objective value, an air/fuel ratio sensor is judged to be abnormal. By this method, only by monitoring the voltage of the sensor cell 32, the generation of the abnormality of the air/ fuel ratio sensor can be detected.
MATSUOKA TOSHIYA
KONDO TOSHIAKI
YAMADA TETSUMASA
KAWAI TAKASHI
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