| 摘要 |
An air-fuel ratio sensor system is diagnosed for the presence/absence of an abnormality on the basis of a reference voltage appearing at one end of a current detecting resistor and a detection voltage appearing at the other end of the current detecting resistor for detecting a current output by an air-fuel ratio sensor. If the reference voltage is outside a normal range and higher than the detection voltage, the air-fuel ratio sensor system is determined to be in a rich abnormality. If the reference voltage Vafon is outside the normal range and lower than the detection voltage, the air-fuel ratio sensor system is determined to be in a lean abnormality. If the reference voltage is within the normal range and a state of the detection voltage being close to a power supply voltage has been continuing for a predetermined period of time, the air-fuel ratio sensor system is determined to be in a circuit-shorting. If the reference voltage is within the normal range and a state of the detection voltage being equal to the reference voltage has been continuing at least for the predetermined period of time, the air-fuel ratio sensor system is determined to be in a circuit-breakage.
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