Importance

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Abstract

An exhaust emission control system for an internal combustion engine, having a catalyst provided in an exhaust system of the engine for purifying exhaust gases, and a NOx removing device provided downstream of the catalyst for absorbing NOx contained in the exhaust gases in an exhaust lean condition, is disclosed. A first oxygen concentration sensor is provided between the catalyst and the NOx removing device, and a second oxygen concentration sensor is provided downstream of the NOx removing device. A first time period, which is an elapsed time period from the time the output from the first oxygen concentration sensor has reached a first reference value after switching the air-fuel ratio from the lean region to the rich region, is measured. A second time period, which is an elapsed time period from the time the output from the first oxygen concentration sensor has reached a second reference value corresponding to a richer air-fuel ratio with respect to the first reference value, is measured. It is determined according to the first and second time periods and the output from the second oxygen concentration sensor that the NOx removing devices is normal or deteriorated.

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First Claim