Malfunction diagnosis device for exhaust gas purification device of internal combustion engine

Abstract

The invention relates to a malfunction diagnosis device for an exhaust gas purification catalyst of an engine. The device calculates, on the basis of a parameter of the engine other than an output value of a NOx concentration sensor, (a) an estimated NO concentration of the exhaust gas and expected to flow out from the catalyst and (b) an estimated particular component concentration of the exhaust gas and expected to flow out from the catalyst. The device calculates an expected output value such that the expected output value calculated when the influence parameter value is a first value is smaller than the expected output value calculated when the influence parameter value is a second value, the expected output value being an output value which is expected to be output from the sensor when the exhaust gas, which includes NO having the expected NO concentration and the particular component having the expected particular component concentration, reaches the sensor. The device diagnoses that a malfunction occurs in the catalyst when the output value of the sensor is equal to or larger than a malfunction diagnosis threshold defined on the basis of the expected output value.