A method for checking the tightness of a tank system, especially of a tank-venting system of a vehicle, wherein an overpressure or an underpressure relative to the atmospheric pressure is introduced into the tank system by means of a pressure source and the time-dependent trace of at least one operating characteristic variable of the pressure source is detected when introducing the overpressure/underpressure up to reaching a first time point or a first pressure level and is compared to a time-dependent trace of the operating characteristic value up to this time point or up to this pressure level (expected diagnostic trace), the time-dependent trace being expected in the case of a tight tank system, and, a conclusion as to non-tightness is drawn when the detected time-dependent trace deviates from the expected diagnostic trace by at least a pregiven value, characterized in that, when a deviation is determined, the overpressure/underpressure in the tank-venting system is further increased up to reaching a second time point or a second pressure level; the operating characteristic value of the pressure source continues to be detected and compared to an expected further time-dependent trace of the operating variable in the case of a tight system (further diagnostic trace) and, a fault announcement is only then outputted when the detected time-dependent trace during the introduction of the further overpressure/underpressure deviates from the further diagnostic trace.