Diagnostic device

A diagnostic device for use with an exhaust purification system including an oxidation catalyst to oxidize hydrocarbons contained in exhaust gas, and a filter downstream of the oxidation catalyst with respect to a flowing direction of the exhaust gas to accumulate particulate matter contained in the exhaust gas, and to undergo forced regeneration by hydrocarbons supplied to the oxidation catalyst to burn and remove the particulate matter accumulated in the filter, the diagnostic device comprising:

• a first temperature sensor to detect a temperature of the oxidation catalyst;
• a controller configured to
• calculate a thermal history time of the oxidation catalyst, which is a time period during which a heat load is received relative to a predetermined set temperature during a regeneration interval of the oxidation catalyst,
• calculate a first degradation degree of the oxidation catalyst based on the thermal history time,
• calculate a first quantity of heat generated in the oxidation catalyst based on the temperature 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
• perform a diagnosis as to degradation of the oxidation catalyst based on at least one of the first degradation degree and the second degradation degree.