Exhaust emission control system for internal combustion engine

摘要

An exhaust emission control system for an internal combustion engine having nitrogen oxides removing device provided in an exhaust system of the engine for removing nitrogen oxides in exhaust gases and an oxygen concentration sensor provided downstream of the nitrogen oxides removing device for detecting the concentration of oxygen in the exhaust gases. Deterioration of the nitrogen oxides removing device is determined on the basis of an output of the oxygen concentration sensor after the air-fuel ratio has been changed from the lean region to a rich region with respect to the stoichiometric ratio. Abnormality of the oxygen concentration sensor is determined on the basis of an output of the oxygen concentration sensor during a period in which the air-fuel ratio is kept in the rich region with respect to the stoichiometric ratio immediately after execution of the deterioration determination of the nitrogen oxides removing device.