A method for testing the function of a fuel tank system of an internal combustion engine, the fuel tank system, whereby a gas transport device is operated and a shutoff valve is actuated by means of PWM, whereby the shutoff valve is opened and closed multiple times according to a PWM signal for implementing a defined opening state, a pressure oscillation of the purge gas, which results due to the corresponding opening and closing movements of the shutoff valve, being ascertained and evaluated with the aid of the pressure sensor according to a frequency diagnosis, and a distinction being made between an operability and a malfunction, based on the result of the evaluation.