Diagnosis system of internal combustion engine

An electronic control unit (ECU) of 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, performing a fuel cut control by stopping or decreasing a feed of fuel to a combustion chamber, 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 ECU is configured to:

• calculate a first characteristic of change of air-fuel ratio at the time 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,
• calculate a second characteristic of change of air-fuel ratio at the time 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, after an end of the fuel cut control, based on an output air-fuel ratio output from the air-fuel ratio sensor, and
• judge if a state of the air-fuel ratio sensor is normal or abnormal based on the first characteristic of change of air-fuel ratio and, if a judgment cannot be made based on the first characteristic of change of air-fuel ratio, judge if the state of the air-fuel ratio sensor is normal or abnormal based on the second characteristic of change of air-fuel ratio.