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.