Catalyst deterioration detecting apparatus for internal combustion engine

Abstract

An apparatus for detecting deterioration of a catalyst (,) for an internal combustion engine which can ensure high reliability for the decision as to deterioration of the catalyst by preventing positively erroneous determination of the deteriorated state of the catalyst due to variations of operating state. The apparatus includes first and second air-fuel ratio sensors (,) provided upstream and downstream of the catalyst (,), respectively, a deterioration parameter arithmetic means (,) for arithmetically determining a deterioration parameter (P) for decision as to deterioration of the catalyst (,) on the basis of comparison between first and second detection signals (D,, D,) of the first and second air-fuel ratio sensors (,), respectively, an event frequency counting means (,) for counting occurrences of the deterioration parameter (P) in each of plural distribution fields prepared in dependence on magnitudes of arithmetically determined values of the deterioration parameter (P) as event frequency count data (N,to N,), respectively, a statistic value arithmetic means (,) for arithmetically determining a statistic value (Q) on the basis of the event frequency count data (N,to N,) registered in the distribution fields, and a catalyst deterioration decision means (,A) for deciding whether the catalyst (,) is deteriorated or not on the basis of the statistic value (Q).