Patent ID: 7198952

Claim:
A catalyst deterioration detecting apparatus for an internal combustion engine, comprising: an upstream side catalyst disposed in an exhaust passage of the internal combustion engine; a downstream side catalyst disposed downstream of the upstream side catalyst; a first oxygen sensor that detects an oxygen concentration of an exhaust gas which flows out from the upstream side catalyst; and a controller that: detects a maximum stored oxygen state of the upstream side catalyst, from which exhaust gas having an excess amount of oxygen flows out downstream, based on a detection value of the first oxygen sensor; detects a minimum stored oxygen state of the upstream side catalyst, from which exhaust gas having a shortage of oxygen flows out downstream, based on the detection value of the first oxygen sensor; forces an air-fuel ratio, of a mixture to be supplied to the internal combustion engine, to be rich after the upstream side catalyst has reached the maximum stored oxygen state until the upstream side catalyst has reached the minimum stored oxygen state; forces the air-fuel ratio, of the mixture to be supplied to the internal combustion engine, to be lean after the upstream side catalyst has reached the minimum stored oxygen state until the upstream side catalyst has reached the maximum stored oxygen state; selectively detects, as an oxygen storage capacity of the upstream side catalyst, an amount of oxygen discharged by the upstream side catalyst during a process in which the upstream side catalyst shifts from the maximum stored oxygen state to the minimum stored oxygen state and an amount of oxygen stored by the upstream side catalyst during a process in which the upstream side catalyst shifts from the minimum stored oxygen state to the maximum stored oxygen state; determines whether the downstream side catalyst is in a predetermined state to store both oxygen equal to, or greater than, a predetermined amount and discharge oxygen equal to, or greater than, a predetermined amount; and determines a deterioration state of the upstream side catalyst based on the oxygen storage capacity, wherein a determination of the deterioration state of the upstream side catalyst is allowed to start only when the downstream side catalyst is in the predetermined state.