Failure diagnostic apparatus and method for an air-fuel ratio sensor

摘要

A failure diagnostic apparatus is provided with an offset power source for offsetting a ground-side voltage of an air-fuel ratio sensor, an activation state judging unit for judging whether the air-fuel ratio sensor is active, a failure diagnosing unit for judging for a failure from an offset-added output signal of the air-fuel ratio sensor in a period when the activation state judging unit judges that the air-fuel ratio sensor is active, an input resistance switching unit for switching the level of an input signal from the air-fuel ratio sensor when the failure diagnosing unit has detected a failure in the air-fuel ratio sensor, and a failure state judging unit for determining a type of failure of the air-fuel ratio sensor on the basis of a voltage level obtained when the input resistance switching unit has switched the input signal level.