Diagnostic device

A diagnostic device for use with an exhaust purification system including an oxidation catalyst arranged to oxidize hydrocarbons contained in exhaust gas, a filter arranged downstream of the oxidation catalyst with respect to an gas exhaust flowing direction and configured to collect particulate matter contained in the exhaust gas, and forced regeneration unit configured to perform a forced regeneration of supplying the hydrocarbons to the oxidation catalyst and burning and removing particulate matter accumulated in the filter, the diagnostic device comprising:

• a first temperature sensor to detect a temperature of the oxidation catalyst;
• a first degradation degree calculation unit configured to convert a cumulative time of the temperature entered from the first temperature sensor to a thermal history time for a specified set temperature in a period from an end of a forced regeneration to a start of a next forced regeneration, and calculate a first degradation degree of the oxidation catalyst based on the thermal history time;
• a second degradation degree calculation unit configured to calculate a first quantity of heat generated in the oxidation catalyst based on the temperature entered from the first temperature sensor in a forced regeneration period, and calculate a second degradation degree of the oxidation catalyst based on the first quantity of heat generated;
• and
• a first diagnosis unit configured to perform a diagnosis as to degradation of the oxidation catalyst based on at least one of the first degradation degree entered from the first degradation degree calculation unit and the second degradation degree entered from the second degradation degree calculation unit.