A method for operating an internal combustion engine, in which an erroneous lift of an intake valve of a cylinder is directly determined. The lift of an intake valve of at least one cylinder of the internal combustion engine is diagnosed. A value for a specific cylinder which is representative of the air/fuel ratio in the respective cylinder is determined in an exhaust system branch of the internal combustion engine. At least one such value is compared to a predefined value. An erroneous lift of the intake valve is diagnosed as a function of the deviation of this at least one value from the predefined value.