Fault diagnosis apparatus for exhaust gas purification system

A fault diagnosis apparatus for an exhaust gas purification system for diagnosing whether or not the exhaust gas purification system is faulty, the exhaust gas purification system including an SCR filter provided in an exhaust passage of an internal combustion engine and an ammonia supply device that supplies ammonia to said SCR filter, said SCR filter including a filter and an SCR catalyst supported on said filter, said SCR catalyst having a capability of reducing NOx in exhaust gas by using ammonia as reducing agent, and said filter having a function of trapping particulate matter in exhaust gas, comprising:

• an NOx sensor provided in the exhaust passage downstream of said SCR filter;
• an NOx removal rate calculation unit configured to calculate the NOx removal rate with said SCR filter by using a measurement value of said NOx sensor;
• a corrected removal rate calculation unit configured to calculate a corrected removal rate by subtracting a decrease that is determined on the basis of a differential pressure change rate at a time of sensor measurement from the NOx removal rate with said SCR filter calculated by said NOx removal rate calculation unit, said differential pressure change rate being defined as the amount of increase in a converted differential pressure value normalizing the differential pressure of the exhaust gas across said SCR filter by the exhaust gas rate per unit increase in a filter PM deposition amount defined as the amount of particulate matter deposited in said SCR filter that is estimated on the basis of a parameter other than said converted differential pressure value, and said time of sensor measurement being defined as the time at which the measurement value of said NOx sensor used in calculation of the NOx removal rate by said NOx removal rate calculation unit is obtained;
• and
• a determination unit configured to determine that said exhaust gas purification system is faulty, if said corrected removal rate calculated by said corrected removal rate calculation unit is equal to or lower than a predetermined criterion removal rate that is determined on the assumption that particulate matter is not deposited in said SCR filter,
• wherein said corrected removal rate calculation unit makes said decrease larger in the case where said differential pressure change rate at said time of sensor measurement is lower than a predetermined threshold than in the case where said differential pressure change rate at said time of sensor measurement is equal to or higher than said predetermined threshold.