Failure diagnostic apparatus and method for air-fuel ratio detecting device

A failure diagnostic apparatus for air-fuel ratio detecting device, comprising:

• an exhaust emission purifying device provided in an exhaust passage of an internal combustion engine;
• an air-fuel ratio detecting unit that detects an air-fuel ratio of an exhaust gas from said exhaust emission purifying device;
• a first determination unit that determines that the internal combustion engine has been operated in a first operating state, in which an air-fuel ratio of the exhaust gas in a vicinity of said exhaust emission purifying device is estimated to be rich, for a longer period of time than a first predetermined period of time;
• a second determination unit that determines that the internal combustion engine has been operated in a second operating state, in which an air-fuel ratio of the exhaust gas in a vicinity of said exhaust emission purifying device is estimated to be lean, for a longer period of time than a second predetermined period of time;
• an air-fuel ratio variation calculating unit that calculates a variation in the air-fuel ratio outputted from said air-fuel ratio detecting unit until said first determination unit and said second determination unit make the determinations;
• and
• a failure diagnosis device that determines that said air-fuel ratio detecting unit has failed if said first determination unit and said second determination unit make the determinations and if it is determined that the variation in the air-fuel ratio calculated by said air-fuel ratio variation calculating unit is less than a predetermined value.