Patent attributes
A catalyst deterioration determination system effectively prevents erroneous determination caused by increase of the H2 component in exhaust gas and improves accuracy of determination. A LAF sensor and an O2 sensor are provided on the upstream side and downstream side of a three-way catalyst. Deterioration of the three-way catalyst is determined on the basis of an output of the O2 sensor, which is detected during air-fuel ratio switching control in which an air-fuel ratio is alternately switched to the rich side and lean side of the stoichiometric air-fuel ratio according to a detected equivalent ratio detected by the LAF sensor, on the basis of a result of comparison between an oxygen inflow into the three-way catalyst and a lean side-limiting value and a rich side-limiting value which are target values of the oxygen inflow. Fuel injection timing retardation control and/or divided fuel injection control are prohibited during the deterioration determination.
