Patent ID: 6839637

Claim:
An exhaust emission control system for an internal combustion engine having an exhaust system, comprising: NOx removing means provided in said exhaust system of said engine for removing NOx contained in exhaust gases when the air-fuel ratio of an air-fuel mixture to be supplied to said engine is set to a value which is leaner than the stoichiometric ratio; an oxygen concentration sensor provided in said exhaust system, for detecting an oxygen concentration in the exhaust gases; air-fuel ratio changing means for changing the air-fuel ratio of the air-fuel mixture to be supplied to said engine from a value which is leaner than the stoichiometric ratio to a value which is richer than the stoichiometric ratio; and sulfur oxide determining means for determining whether or not a sulfur oxide concentration in the exhaust gases is high according to a comparison between a predetermined transient time period and a transient time period, said transient time period being determined based on the oxygen concentration detected by said oxygen concentration sensor after the air-fuel ratio is changed by said air-fuel ratio changing means.