Actual values of a load variable are detected by a sensor during active model trimming under different valve pressure conditions, and a model of estimated values is created. Trimming values of a model parameter are determined depending on detected and modeled values such that when the model parameter is trimmed, the modeled values approximate the detected values. A function is determined depending on the different pressure conditions and approximated to the trimming values. At different pressure conditions first lambda control values are detected. A first controller value fitted function is determined depending on the pressure conditions and approximated. When the model trimming mode is not active, second lambda control values are detected at different pressure conditions. A second controller value fitted function is determined depending on the pressure conditions and is approximated. An error of the intake system is detected depending on the fitted functions.