Importance

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Abstract

A catalyst deterioration-determining system for an internal combustion engine has a catalyst arranged in the exhaust system, for purifying exhaust gases emitted from the engine, an oxygen concentration sensor arranged in the exhaust system at a location downstream of the catalyst, for detecting the concentration of oxygen present in the exhaust gases, and an ECU which controls the air-fuel ratio of an air-fuel mixture supplied to the engine in a feedback manner in response to the output from the oxygen concentration sensor. Determination of deterioration of the catalyst is carried out based on the output from the oxygen concentration sensor during execution of air-fuel ratio feedback control. An inversion period parameter representative of a repetition period of change in the output from the oxygen concentration sensor over a predetermined diagnosis period is measured, and a determination parameter is calculated by averaging values of the inversion period parameter measured over different diagnosis periods as the predetermined diagnosis period. The determination of deterioration of the catalyst is carried out based on the determination parameter.

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First Claim