A method of providing a model which provides air/fuel ratio at a point of an air/fuel sensor located in an exhaust system of an engine includes a) providing a first model which provides a first air/fuel ratio which is outlet from an exhaust manifold of the engine; b) measuring or estimating volumetric flow through the exhaust; c) applying a transfer function to the first air/fuel ratio to provide a model of the air/fuel ratio at the sensor. The transfer function includes a first order filter and is dependent on the volumetric flow. The transfer function has a time constant equivalent to a filter coefficient which is 1/time constant which is determined based on the flow rate.