Patent ID: 6840035

Claim:
An exhaust emission control system for a multiple cylinder internal combustion engine, comprising: a plurality of upstream exhaust passages provided for respective ones of a plurality of cylinders of the internal combustion engine; a downstream exhaust passage formed by joining said plurality of upstream exhaust passages; a plurality of upstream catalytic converters disposed in respective ones of said plurality of upstream exhaust passages; a downstream catalytic converter disposed in said downstream exhaust passage; at least one upstream exhaust sensor provided for said upstream exhaust passages and positioned downstream of said upstream catalytic converters to detect an exhaust air fuel ratio; an exhaust air-fuel ratio changing device that switches an exhaust air-fuel ratio of exhaust gas flowing into one of said plurality of upstream exhaust passages and an exhaust air-fuel ratio of exhaust gas flowing into another one of said plurality of upstream exhaust passages to a rich air-fuel ratio and to a lean air-fuel ratio alternately on a predetermined cycle longer than each of combustion cycles of the internal combustion engine; and a deterioration determination device that determines whether said upstream catalytic converters have deteriorated according to a change in the exhaust air-fuel ratio detected by said upstream exhaust sensor.