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

A malfunction diagnosis device for an exhaust gas purification catalyst of an internal combustion engine, comprising:

• an exhaust passage;
• an exhaust gas purification catalyst provided in the exhaust passage for reducing nitrogen monoxide included in exhaust gas by ammonia to purify the nitrogen monoxide;
• and
• a NOx concentration sensor provided in the exhaust passage downstream of the exhaust gas purification catalyst for detecting a NOx concentration of the exhaust gas,
• the NOx concentration sensor including a diffusion layer made of porous material, through which the exhaust gas can pass, and an electrode part for reducing nitrogen monoxide included in the exhaust gas, which has passed through the diffusion layer to convert the nitrogen monoxide to nitrogen and outputting an output value depending on an amount of reduction of the nitrogen monoxide at the electrode part,
• the malfunction diagnosis device comprising a malfunction diagnosis section for diagnosing a malfunction of the exhaust gas purification catalyst,
• wherein an amount of nitrogen monoxide, produced from a particular component included in the exhaust gas when the exhaust gas passes through the diffusion layer, is a first amount when an influence parameter value is a first value, the influence parameter value being a value of an influence parameter which influences a reaction of the particular component when the particular component passes through the diffusion layer, the reaction of the particular component changing an amount of nitrogen monoxide reaching the electrode part,
• an amount of nitrogen monoxide, produced from the particular component when the particular component passes through the diffusion layer, is a second amount larger than the first amount when the influence parameter value is a second value,
• the malfunction diagnosis part is programmed:
• to calculate, on the basis of a parameter of the engine other than the output value of the NOx concentration sensor, (a) an estimated nitrogen monoxide concentration which is a concentration of nitrogen monoxide included in the exhaust gas and expected to flow out from the exhaust gas purification catalyst and (b) an estimated particular component concentration which is a concentration of the particular component included in the exhaust gas and expected to flow out from the exhaust gas purification catalyst;
• to calculate an expected output value such that the expected output value calculated when the influence parameter value is the first value is smaller than the expected output value calculated when the influence parameter value is the second value, the expected output value being an output value which is expected to be output from the NOx concentration sensor when the exhaust gas, which includes nitrogen monoxide having the expected nitrogen monoxide concentration and the particular component having the expected particular component concentration, reaches the NOx concentration sensor;
• and
• to diagnose that a malfunction occurs in the exhaust gas purification catalyst when the output value of the NOx concentration sensor is equal to or larger than a malfunction diagnosis threshold defined on the basis of the expected output value.