Method for controlling engine exhaust gas system

A method for controlling an engine exhaust gas system includes the steps of detecting an air-fuel ratio of exhaust gas discharged from an internal engine by an oxygen sensor, controlling the exhaust gas to near the stoichiometric air-fuel ratio by a closed-loop control referring to a resultant signal, and leading the exhaust gas to a three-way catalyst to treat nitrogen oxide, hydrocarbon and carbon monoxide. A nitrogen oxide sensor is provided downstream of the three-way catalyst, and a control value of air-fuel ratio by a closed loop control by the oxygen sensor is corrected to set the nitrogen oxide concentration to a predetermined value in accordance with an output of the nitrogen oxide sensor. The deterioration of the three-way catalyst and/or oxygen sensor is detected by comparing a change in control corrected value for the air-fuel ratio of closed loop control referring to an output of the nitrogen oxide sensor with a predetermined value.