The disclosure provides a method to operate an engine system. The method begins by determination of a current NOx value. If the current NOx value is greater than a first predetermined NOx target, an air-fuel ratio (AFR) set-point is adjusted by a first predetermined value in towards a rich AFR. Upon detection of the current NOx value below the first predetermined NOx target, a first average of NOx values for a first predetermined time is determined. The AFR set-point is adjusted by a second predetermined value and a second average of NOx values for a second predetermined time is determined. A delta NOx value is determined as a difference between the first average and the second average and compared with a second predetermined NOx target. The AFR set-point is determined when the delta NOx value is below the second predetermined NOx target.