Patent ID: 7536244

Claim:
A failure diagnostic apparatus for an air-fuel ratio sensor, comprising: an air-fuel ratio sensor attached to an exhaust pipe of an internal combustion engine configured to detect an air-fuel ratio from an oxygen concentration of exhaust gas; an offset power source configured to offset a ground-side voltage of the air-fuel ratio sensor; means for judging whether the air-fuel ratio sensor is active; means for judging for a failure from an offset-added output signal of the air-fuel ratio sensor in a period when the means for judging whether the air-fuel ratio sensor is active judges that the air-fuel ratio sensor is active; means for switching a level of an input signal from the air-fuel ratio sensor when the means for judging for the failure has detected a failure in the air-fuel ratio sensor; and means for inputting the input signal from the air-fuel ratio sensor through an A/D converter and determining whether the air-fuel ratio sensor is in a disconnection state or a ground fault state on the basis of a voltage level obtained when the means for switching a level of the input signal has switched the input signal level; wherein an offset voltage of the offset power source is set higher than a maximum output voltage of the air-fuel ratio sensor and a maximum offset-added output voltage of the air-fuel ratio sensor is set lower than a maximum allowable input voltage of the A/D converter.