Diagnosis system of internal combustion engine

A diagnosis system comprising:

• an internal combustion engine comprising an exhaust purification catalyst arranged in an exhaust passage of the internal combustion engine and being able to store oxygen in inflowing exhaust gas and an air-fuel ratio sensor arranged at a downstream side of the exhaust purification catalyst in a direction of exhaust flow and detecting an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst, stopping or decreasing a feed of fuel to a combustion chamber as fuel cut control, and controlling an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst after the end of the fuel cut control to a rich air-fuel ratio richer than a stoichiometric air-fuel ratio as post reset rich control,
• wherein the diagnosis system further comprises
• an electronic control unit that is configured to control an amount of fuel fed to the internal combustion engine based on output of the air-fuel ratio sensor;
• the electronic control unit is configured to calculate a first change of speed of air-fuel ratio which is a change of speed when the output air-fuel ratio of the air-fuel ratio sensor first passes a first air-fuel ratio region which is a part of an air-fuel ratio region of a stoichiometric air-fuel ratio or more, after an end of the fuel cut control, based on an output air-fuel ratio output from the air-fuel ratio sensor,
• the electronic control unit is further configured to calculate a second change of speed of air-fuel ratio which is a change of speed when the output air-fuel ratio of the air-fuel ratio sensor first passes a second air-fuel ratio region different from the first air-fuel ratio region, based on an output air-fuel ratio output from the air-fuel ratio sensor, and
• the electronic control unit is further configured to diagnose abnormality of the air-fuel ratio sensor based on a corrected change of speed of air-fuel ratio calculated by correcting the calculated first change of speed of air-fuel ratio based on the calculated second change of speed of air-fuel ratio,
• wherein the electronic control unit is further configured to correct the first change of speed of air-fuel ratio so that the corrected change of speed of air-fuel ratio becomes slower the faster the second change of speed of air-fuel ratio, to judge that the air-fuel ratio sensor is abnormal when the corrected change of speed of air-fuel ratio is slower than a speed of change used as reference for abnormality, and to judge that the air-fuel ratio sensor is normal when the first change of speed of air-fuel ratio corrected based on the second change of speed of air-fuel ratio is faster than a speed of change used as reference for abnormality.