Catalyst deterioration diagnosis method and catalyst deterioration diagnosis system

A catalyst deterioration diagnosis system that diagnoses a degree of deterioration of a catalyst into which an exhaust gas from an internal combustion engine including a fuel injection device is introduced, the catalyst deterioration diagnosis system comprising:

• a gas sensor having ammonia interference property, and being configured to be capable of measuring an air-fuel ratio and nitrogen oxide concentration of an exhaust gas that has passed through the catalyst;
• and
• a control device configured to run the internal combustion engine, the control device comprising:
• a fuel injection controller configured to control operation of the fuel injection device;
• an air-fuel ratio determination unit configured to determine whether or not an air-fuel ratio obtained by the gas sensor is larger than a predetermined threshold air-fuel ratio, the predetermined threshold air-fuel ratio being larger than a stoichiometric air-fuel ratio;
• a monitor unit configured to start monitoring of temporary increase of nitrogen oxide concentration to be detected by the gas sensor, and thereby acquire a temporarily increased amount of the nitrogen oxide concentration, the monitor unit being configured to start the monitoring when the fuel injection controller gives a command of restarting fuel injection after a fuel cut to the fuel injection device in a case where a most recent determination result produced by the air-fuel ratio determination unit as to whether or not the air-fuel ratio obtained by the gas sensor is larger than the predetermined threshold air-fuel ratio suggests that the air-fuel ratio obtained by the gas sensor is larger than the predetermined threshold air-fuel ratio;
• and
• an increased-amount determination unit configured to determine whether or not the temporarily increased amount acquired by the monitor unit is larger than a threshold amount.