Method and system for diagnosing deterioration of exhaust emission control catalyst

A deterioration diagnosing system for an exhaust emission control catalyst of an engine, the exhaust emission control catalyst including an HC (carbon hydride) adsorbing part and an oxidation catalyst part, the HC adsorbing part disposed in an exhaust passage of the engine and for adsorbing HC within exhaust gas when a temperature of the HC adsorbing part is lower than an HC dischargeable temperature and discharging the adsorbed HC when the temperature of the HC adsorbing part is higher than the HC dischargeable temperature, the oxidation catalyst part for purifying, by oxidation, the HC discharged from the HC adsorbing part and the HC within the exhaust gas under a high temperature, the system comprising:

• an actual exhaust-emission-control-catalyst temperature parameter detecting module for detecting a parameter correlating with an actual temperature of the exhaust emission control catalyst;
• a deterioration determining module for receiving a detection value from the actual exhaust-emission-control-catalyst temperature parameter detecting module when predetermined diagnosis executing conditions are met, and determining that the exhaust emission control catalyst is deteriorated when the detection value is smaller than a predetermined diagnostic temperature parameter threshold;
• an HC discharge amount calculating module for calculating an amount of discharging HC from the HC adsorbing part;
• and
• a false deterioration determination preventing module for receiving a signal from the HC discharge amount calculating module and preventing false determination of the deterioration determining module that is induced by an increase of the actual exhaust-emission-control-catalyst temperature parameter associated with an increase of the HC discharge amount.