A method for controlling fuel supply for an assembly comprising: a) a spark ignition internal combustion engine; b) an exhaust after-treatment system comprising a three-way catalyst; c) a first lambda sensor arranged upstream of said catalyst, considering a flow direction of exhaust gases in said catalyst; and d) an injection device supplying fuel to said engine; the method comprising the steps of: - controlling said injection device according to a first set-point (λset), indicative of a desired air/fuel ratio during combustion of a mixture of fuel and air in said engine; - determining an air/fuel ratio value (λr) indicative of an actual air/fuel ratio, on the basis of a measurement of said first lambda sensor; - simulating said catalyst by a catalyst model comprising a plurality of cells (Cell1, Cell2,..., Celln), which correspond to respective areas arranged in series in said catalyst along said flow direction and are associated to respective oxygen levels (Ocell1, Ocell2,... Ocelln); said cells comprising a first cell (Cell1) corresponding to a first area of said areas in said catalyst along said flow direction; - estimating said oxygen levels (Ocell1, Ocell2,... Ocelln) respectively in the cells of said catalyst model, at least on the basis of said air/fuel ratio value (λr); the method characterized by further comprising the steps of: - setting a wobbling pattern for said first set-point (λset) so as to operate said engine with alternating lean combustion phases and rich combustion phases; - setting an upper threshold and a lower threshold; - comparing said upper and lower thresholds (S1,S2) with a first parameter (Ocell1) indicative only of the estimated oxygen level of said first cell (Cell1); said wobbling pattern being set so as to bring the engine to the lean combustion phase when said first parameter (Ocell1) becomes lower than said lower threshold and to the rich combustion phase when said first parameter (Ocell1) becomes higher than said upper threshold. a) a spark ignition internal combustion engine; b) an exhaust after-treatment system comprising a three-way catalyst; c) a first lambda sensor arranged upstream of said catalyst, considering a flow direction of exhaust gases in said catalyst; and d) an injection device supplying fuel to said engine; the method comprising the steps of: - controlling said injection device according to a first set-point (λset), indicative of a desired air/fuel ratio during combustion of a mixture of fuel and air in said engine; - determining an air/fuel ratio value (λr) indicative of an actual air/fuel ratio, on the basis of a measurement of said first lambda sensor; - simulating said catalyst by a catalyst model comprising a plurality of cells (Cell1, Cell2,..., Celln), which correspond to respective areas arranged in series in said catalyst along said flow direction and are associated to respective oxygen levels (Ocell1, Ocell2,... Ocelln); said cells comprising a first cell (Cell1) corresponding to a first area of said areas in said catalyst along said flow direction; - estimating said oxygen levels (Ocell1, Ocell2,... Ocelln) respectively in the cells of said catalyst model, at least on the basis of said air/fuel ratio value (λr); - controlling said injection device according to a first set-point (λset), indicative of a desired air/fuel ratio during combustion of a mixture of fuel and air in said engine; - determining an air/fuel ratio value (λr) indicative of an actual air/fuel ratio, on the basis of a measurement of said first lambda sensor; - simulating said catalyst by a catalyst model comprising a plurality of cells (Cell1, Cell2,..., Celln), which correspond to respective areas arranged in series in said catalyst along said flow direction and are associated to respective oxygen levels (Ocell1, Ocell2,... Ocelln); said cells comprising a first cell (Cell1) corresponding to a first area of said areas in said catalyst along said flow direction; - estimating said oxygen levels (Ocell1, Ocell2,... Ocelln) respectively in the cells of said catalyst model, at least on the basis of said air/fuel ratio value (λr); the method characterized by further comprising the steps of: - setting a wobbling pattern for said first set-point (λset) so as to operate said engine with alternating lean combustion phases and rich combustion phases; - setting an upper threshold and a lower threshold; - comparing said upper and lower thresholds (S1,S2) with a first parameter (Ocell1) indicative only of the estimated oxygen level of said first cell (Cell1); - setting a wobbling pattern for said first set-point (λset) so as to operate said engine with alternating lean combustion phases and rich combustion phases; - setting an upper threshold and a lower threshold; - comparing said upper and lower thresholds (S1,S2) with a first parameter (Ocell1) indicative only of the estimated oxygen level of said first cell (Cell1); said wobbling pattern being set so as to bring the engine to the lean combustion phase when said first parameter (Ocell1) becomes lower than said lower threshold and to the rich combustion phase when said first parameter (Ocell1) becomes higher than said upper threshold.