A device and method for testing seal tightness of a fuel tank system of a motor vehicle, wherein excess pressure is built up in the fuel tank system by an electrically operated pump and electric current consumption of the pump is compared to reference values in order to determine whether the seal tightness of the fuel tank system is sufficient, and wherein the fuel tank system includes a filler neck with a locking fuel tank cap. A control line is connected in parallel to the power supply line of the pump, the control line being guided across an electric tank cap switch, which assumes a different switch state when the filler neck is closed properly than when it is not closed properly. In at least one of these lines, an electric resistor is installed, which allows a determination of the switch state of the tank cap switch.