A device for checking the tightness of a tank, and a modular system for a plurality of design variants of such a device

The invention relates to a device (10) for checking the tightness of a tank (12) for storing a fuel (14) for an internal combustion engine, having at least one pressure sensor (34) for detecting a pressure in the tank (12), with at least one flushing channel (30) for guiding rinsing air to an adsorption filter (22) for filtering fuel components from the gas from the tank (12), with at least one valve (36) for fluidly blocking the flushing channel (30) and with a pump (38) for conveying of air into the tank (12), wherein an at least indirectly to the adsorption filter (22) fluidically connectable, and the pressure sensor (34), the flushing channel (30) and the valve (36) (full first module member (46) and the pump 38) comprehensive second module member (50) are provided which reversibly releasably (with the first module member 46) is connectable.