A method of operating an engine system is described in which a back-pressure valve of a low pressure EGR system can be modulated at a lower frequency during an EGR operation, and at a higher frequency when additional exhaust gas sensing is requested. Back-pressure valve modulation can therefore be used to promote exhaust gas mixing while providing EGR. Exhaust gas constituents in the mixed exhaust gas can then be sensed with higher accuracy by downstream exhaust gas sensors.