In a method for diagnosing a valve in a fluid supply line to a suction line of an air system of a combustion engine, the fluid supply line being is part of an exhaust-gas recirculation system or a crankcase ventilation system, when the valve is actuated, a first pressure in the suction line is monitored for change, and an error of the valve is detected if the change of the first pressure lies outside a specific reference range when the valve is actuated.