An injector has a valve pin whose position is adjustable according to the pressure prevailing in a control chamber and which blocks a fluid flow through at least one injection port in a closed position. The injector has a control chamber that is hydraulically coupled to a high-pressure fluid reservoir, a piezo actuator, and a control valve which hydraulically connects and disconnects the control chamber, the piezo actuator acting upon said control valve. A combination of a triggering period for the piezo actuator and electric power that is to be fed to the piezo actuator is varied from a predefined initial combination to a target combination in which a pressure curve is detected that is characteristic of a movement of the control valve out of the closed position thereof without fluid being proportioned through the injection port.