Catalyst deterioration diagnosis apparatus

A catalyst deterioration diagnosis apparatus that diagnoses deterioration of a catalyst converter to which exhaust gas of an engine is introduced, the apparatus comprising:

• an upstream side air-fuel ratio detector that detects an air-fuel ratio of the exhaust gas being introduced to the catalyst converter;
• a downstream side air-fuel ratio detector that detects an air-fuel ratio of the exhaust gas being discharged from the catalyst converter;
• an air-fuel ratio variation integrator that calculates an output variation integrated value of the upstream side air-fuel ratio detector and an output variation integrated value of the downstream side air-fuel ratio detector when a predetermined diagnosis running condition is satisfied;
• and
• a determiner that determines catalyst deterioration of the catalyst converter when the output variation integrated value of the upstream side air-fuel ratio detector exceeds a predetermined integration ending threshold, and further when a ratio of the output variation integrated value of the downstream side air-fuel ratio detector to the output variation integrated value of the upstream side air-fuel ratio detector exceeds a predetermined determination threshold
• wherein, when the diagnosis running condition becomes unsatisfied before the output variation integrated value of the upstream side air-fuel ratio detector reaches the integration ending threshold, the air-fuel ratio variation integrator holds the current output variation integrated value of the upstream side air-fuel ratio detector and the current output variation integrated value of the downstream side air-fuel ratio detector and, when the diagnosis running condition is satisfied later again, the air-fuel ratio variation integrator resumes integration of the output variation integrated values using the held output variation integrated value of the upstream side air-fuel ratio detector and the held output variation integrated value of the downstream side air-fuel ratio detector as initial values.