Patent ID: 8087231

Claim:
A deterioration-determination apparatus for an exhaust gas purifying system that includes a catalyst in an exhaust passage of an engine for purifying exhaust gas, an upstream sensor and a downstream sensor that detect an air-fuel ratio or a rich/lean condition of the exhaust gas, the upstream sensor being disposed upstream of the catalyst, the downstream sensor being disposed downstream of the catalyst, the deterioration-determination apparatus comprising: a feedback control means for performing a feedback control in which a fuel injection quantity is feedback-corrected based on an output of the upstream sensor so that an air-fuel ratio upstream of the catalyst tends towards a target air-fuel ratio; a sub-feedback control means for adjusting (a) the fuel injection quantity or (b) the main feedback control based on an output of the downstream sensor; and a deterioration-determination means for determining whether a purifying capacity of the exhaust gas purifying system including the catalyst, the upstream sensor and the downstream sensor is deteriorated based on a behavior of an output of the downstream sensor into which a fuel disturbance is inputted during operation of the main feedback control and the sub-feedback control, wherein the deterioration-determination means sets a target air-fuel ratio and an input time period of the fuel disturbance so that a discharge quantity of a specified gas component becomes substantially maximum in inputting the fuel disturbance.