A method for an engine system, said engine system including an engine and an exhaust system, said exhaust system including an outlet exhaust manifold, said method comprising providing a model which provides the parameter of the air/fuel (A/F) ratio (AFR3) at a location in the exhaust system of said engine at a point where an air/fuel sensor is located, said method including the following steps; a) providing a first model which provides the air/fuel ratio which is outlet from the exhaust manifold of said engine (AFR1); b) measuring or estimating a volumetric flow rate through the exhaust; c) applying a transfer function to said A/F ratio (AFR1) determined by said first model to provide a model of the A/F ratio at said location of said sensor (AFR3), said transfer function including a first order (lag) filter, said transfer function being dependent on said volumetric flow determined from b), said transfer function regarded as having a time constant τ, or filter coefficient of K where K=1/τ; said time constant τ or filter coefficient being determined dependent on flow rate, amending the value of the determined time constant τ or filter coefficient K, by applying a correction factor; said correction factor being determined or updated based on the following further steps: d) measuring the A/F ratio (AFR4) at said point by said A/F sensor; e) over a time period, comparing the value of the modelled A/F ratio (AFR3) at the sensor location computed from step c) with the value from the actual A/F sensor(AFR4); f) determining or amending one or more correction factors (cf, CR1) to filter coefficients (K) or time constants (τ) of said applied transfer function based on said comparison g) using said model in the control of said system, wherein step e) comprises comparing a gradient of the value of the modelled A/F ratio at the sensor location (AFR3) with respect to time with a gradient of a value of A/f ratio from the actual A/F sensor (AFR4), and. including determining the ratio of said gradients and determining said correction factor dependent on said ratio. a) providing a first model which provides the air/fuel ratio which is outlet from the exhaust manifold of said engine (AFR1); b) measuring or estimating a volumetric flow rate through the exhaust; c) applying a transfer function to said A/F ratio (AFR1) determined by said first model to provide a model of the A/F ratio at said location of said sensor (AFR3), said transfer function including a first order (lag) filter, said transfer function being dependent on said volumetric flow determined from b), said transfer function regarded as having a time constant τ, or filter coefficient of K where K=1/τ; said time constant τ or filter coefficient being determined dependent on flow rate, amending the value of the determined time constant τ or filter coefficient K, by applying a correction factor; said correction factor being determined or updated based on the following further steps: d) measuring the A/F ratio (AFR4) at said point by said A/F sensor; e) over a time period, comparing the value of the modelled A/F ratio (AFR3) at the sensor location computed from step c) with the value from the actual A/F sensor(AFR4); f) determining or amending one or more correction factors (cf, CR1) to filter coefficients (K) or time constants (τ) of said applied transfer function based on said comparison g) using said model in the control of said system, wherein step e) comprises comparing a gradient of the value of the modelled A/F ratio at the sensor location (AFR3) with respect to time with a gradient of a value of A/f ratio from the actual A/F sensor (AFR4), and. including determining the ratio of said gradients and determining said correction factor dependent on said ratio. <EOS>