A method for operating a metering device for a liquid additive includes providing the metering device with at least one pump having a movable pump element carrying out pumping movements to pump the liquid additive and at least one injector connected through a pressure line to a pressure side of the pump and being opened to meter the liquid additive. The injector is opened in a step a). In a step b), the liquid additive is then metered and the pumping movements are counted during metering. In a step c), the injector is then closed. In a step d), the number of pumping movements ascertained in step b) are then compared with the opening time of the injector between step a) and step c) in order to carry out a diagnosis of the operation of the metering device. A metering device and a motor vehicle are also provided.