A procedure to diagnose a fuel tank ventilation system of a motor vehicle and a device to implement the procedure are proposed. The fuel tank ventilation system contains at least one first ventilation pathway discharging into the air intake area of the internal combustion engine downstream behind a supercharger as well as at least one second ventilation pathway discharging into the air intake area upstream in front of the supercharger. Provision is made for a diagnosis with at least two partial diagnoses, whereby the first partial diagnosis pertaining to the first ventilation pathway is implemented, if the intake manifold pressure in the discharge area of the first ventilation pathway is smaller than the fuel tank system pressure; and the second partial diagnosis pertaining to the second ventilation pathway is implemented, if the intake manifold pressure in the discharge area of the first ventilation pathway is greater than the fuel tank system pressure. An error signal is provided, if at least one of the partial diagnoses generates a partial error signal.