| 摘要 |
An air-fuel ratio sensor has an exhaust-side electrode and an atmosphere-side electrode. The atmosphere-side electrode is disposed in an atmosphere layer that communicates with the atmosphere. Since the air-fuel ratio sensor is disposed in an exhaust passage, exhaust gas enters the atmosphere layer if a sensor crack occurs. Ordinarily, forward voltage is applied to the air-fuel ratio sensor to obtain an output thereof in accordance with the air-fuel ratio. Immediately after the applied voltage is switched to reverse voltage, sensor current i 1 in accordance with the impedance flows, regardless of the presence/absence of a sensor crack. After that, the sensor current converges to a value i 2 that is in accordance with the oxygen concentration in the atmosphere layer. The presence/absence of a sensor crack is determined by comparing the value obtained by correcting i 2 by i 1 , with a criterion value.
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