An air/fuel ratio controller (6) and a method that uses an upstream control loop to maintain a given optimum air/fuel ratio (λopt), whereas the optimum air/fuel ratio (λopt) is determined in the controller (6) in an downstream control loop by adding incremental offset (Δλ) to the air/fuel ratio set-point (λSP) of an upstream control loop while monitoring a NOx sensor (10) output. The air/fuel ratio set-points (λSP) at two turning points (SP1, SP2) in the NOx sensor (10) output are used to calculate a new optimum air/fuel ratio set-point (λopt) as mean value of the air/fuel ratio set-points (λSP1, λSP2) at the turning points (SP1, SP2).