Diagnostic method and apparatus for gas sensor

A gas sensor diagnostic apparatus adapted to diagnose whether a gas sensor is in an abnormal state or not on the basis of a detection signal outputted by the gas sensor exposed in an exhaust gas exhausted from an internal combustion engine, the detection signal representing a concentration of a specific gas component in the exhaust gas, the gas sensor diagnostic apparatus comprising:

• a target air-fuel-ratio reversal number counting section configured to count the reversal number of times that a target air-fuel ratio for an air-fuel mixture to be supplied to the internal combustion engine reverses from a rich side to a lean side or from the lean side to the rich side through a specific air-fuel ratio defined as a boundary of the rich and lean sides;
• a detection signal obtaining section configured to obtain the detection signal of the gas sensor at constant time intervals during a diagnosis period which is a period between a timing when the reversal number of times starts to be counted and a timing when the reversal number of times reaches a predetermined number of times;
• a moderated signal calculating section configured to calculate
• a first moderated signal by applying a first moderation calculation to the obtained detection signal, and
• a second moderated signal by applying a second moderation calculation different from the first moderation calculation, to the obtained detection signal;
• a deviation calculating section configured to calculate a deviation between the calculated first moderated signal and second moderated signal;
• and
• an abnormality diagnosing section configured to determine whether the gas sensor is in the abnormal state or not on the basis of the deviation obtained during the diagnosis period.