1. A method for operating an internal combustion engine having an air system, the method comprising the steps of: computing at least one physical quantity of said air system from several input quantities with the aid of a physical model of said air system wherein said at least one physical quantity is not one of said input quantities; comparing said at least one physical quantity to a measured value for said at least one physical quantity; and, monitoring one of said input quantities or a model internal quantity of said physical model in dependence upon a deviation between the computed value and said measured value.