Patent ID: 7021044

Claim:
A catalyst-deterioration diagnostic apparatus for diagnosing a deterioration state of a catalyst used for eliminating noxious substances contained in engine exhaust gas comprising: an indexer configured to obtain a value of an index which is used for deciding the deterioration state of the catalyst: a catalyst state estimator operative to estimate a state of said catalyst at a time at which said indexer has obtained the index value, as to a physical quantity which affects a catalytic action of said catalyst; a correction element operative to correct said index value obtained by said indexer, to a value in a standard state of said catalyst previously set as to the physical quantity, by the use of the estimated result of said catalyst state estimator; a mechanism operative to suspend a determination of the deterioration state of the catalyst if the value of the standard state value is outside a stored predetermined range; and a decision element endowed with a preset criterion value to decide said deterioration state of said catalyst by comparing the index value corrected by said correction element, with the criterion value, wherein said catalyst state estimator includes an operating-situation detected operative to detect a value of that state variable of the engine which correlates with said physical quantity; a memory operative to store therein correspondence information which indicate correlations between values of the state variable and those of said physical quantity; and an arithmetic element operative to determine a value of said physical quantity by referring to the correspondence information on the basis of the detected result of said operating-situation detector.