Exhaust gas purifying method for lean burn engines

Exhaust gas purification process comprises determining the concentration of the NOx compounds in the form of a NOx model value downstream of the NOx storage catalyst; measuring the concentration of the NOx compounds in the form of a NOx measured value downstream of the catalyst in the exhaust gas pipe; and determining the difference between the NOx measured value and the NOx model value and using to correct the condition parameter in the model. Preferred Features: The NOx measured value or the NOx model value is low-pass filtered before forming the difference. The degree of charge of the catalyst is used as the condition parameter. The difference is coupled with a conversion factor to produce a correction parameter for the condition parameter.