DEVICE FOR DIAGNOSING CAUSES OF DECREASES IN NOx CONVERSION EFFICIENCY

There is provided an apparatus for diagnosing causes of NO X conversion efficiency degradation capable of an on-board diagnostics for specifying a cause of NO X conversion efficiency degradation. The apparatus for diagnosing causes of NO X conversion efficiency degradation includes: a diagnosis exhaust-pipe fuel injection control unit (2) that performs control such that exhaust-pipe fuel injection is performed for diagnosis purposes when a NO X conversion efficiency is degraded; a NO X purification device failure specifying unit (5) that specifies a cause of NO X conversion efficiency degradation as a NO X purification device failure if a difference between an actual heat generation amount and a theoretical heat generation amount in the oxidizing catalyst device at the time of the exhaust-pipe fuel injection is within a threshold value; and an oxidizing catalyst device failure specifying unit (6) that specifies a cause of NO X conversion efficiency degradation as an oxidizing catalyst device failure if a difference between an actual heat generation amount and a theoretical heat generation amount in the oxidizing catalyst device at the time of exhaust-pipe fuel injection exceeds a threshold value.