An association unit is designed to determine cylinder-individual lambda signals on the basis of a lambda probe signal and to determine lambda deviation signals for the respective cylinders based on the lambda signals in relation to an averaged lambda signal. Furthermore, an observer has a sensor model of the lambda probe that is arranged in a feedback branch. The lambda deviation signals are fed to the input side and observer output quantities in relation to the respective cylinder are representative of the injection characteristics deviations from predetermined injection characteristics. A parameter detection unit impresses a predetermined interference pattern from cylinder-individual mixture deviations. It further changes at least one sensor model parameter as a detection parameter in response to the respectively predetermined interference pattern for as long as the observer output quantities represent the portion of the interference pattern associated with the cylinders thereof in a predetermined manner.