Catalyst deterioration diagnosis system and catalyst deterioration diagnosis method

A system of diagnosing a degree of deterioration of a catalyst purifying an exhaust gas emitted from an internal combustion engine mounted in a vehicle, the catalyst including a precious metal component and a ceramic part, the system comprising:

• a drive control element configured and disposed to control intake air and a fuel injection in the internal combustion engine, thereby controlling a drive state of the internal combustion engine;
• an NOx sensor provided on a downstream side with respect to the catalyst in an exhaust path of the exhaust gas;
• a diagnosis element configured and disposed to compare at least a diagnosis object value corresponding to an output from the NOx sensor and a predetermined diagnosis threshold value, thereby diagnosing a degree of deterioration of the catalyst;
• and
• a storage element storing the diagnosis threshold value, wherein
• the diagnosis threshold value is previously determined based on a correlationship between the diagnosis object value and at least a total weight value of NOx and THC in the exhaust gas through the catalyst in a state where the vehicle travels under a constant threshold value setting condition, and
• the diagnosis element diagnoses that a deterioration occurs in the precious metal component in the catalyst when the diagnosis object value obtained while the vehicle travels under a diagnosable condition exceeds the diagnosis threshold value.