NOVELTY - The method involves conducting a defined quantity of anode gas to an anode, and conducting a defined quantity of cathode gas to a cathode, where defined voltage is set at a defined current consumption and a defined power provision. A fuel cell is operated in a cathode gas depletion state to increase the waste heat of the fuel cell for a defined, temporally limited duration. The depletion state is achieved by reducing the mass flow of cathode gas at a constant power provision. The mass flow of cathode gas is reduced down to a range, which is equal to or greater than a stoichiometric range.    USE - Method for operating a fuel cell system of a vehicle i.e. mid-range vehicle.    ADVANTAGE - The method enables operating the fuel cell in the cathode gas depletion state to increase the waste heat of the fuel cell for the defined, temporally limited duration, thus providing an improved thermal control of the fuel cell even in the case of cold starting or after operation in the partial load mode. The method enables maintaining the system at sufficient temperature without the need to provide additional assemblies such as an auxiliary electric heater, so that the additionally consumed fuel can be reduced, and an operating point set at the gas side can be maintained, thus maintaining the parasitic power. The method enables improving the consumption of the vehicle operated using the fuel cell, and saving the fuel required by auxiliary electric heaters, so that the parasitic powers can be reduced or minimized. The method enables using short flushing pulses to remove any quantities of liquid accumulated in the fuel cell or in a fuel cell stack from the system, so that the system can be operated at any time, the electrical power emission is maintained, and the current can be reduced during the flushing phases.    DETAILED DESCRIPTION - The cathode gas is selected from oxygen containing gases i.e. air, and the anode gases are selected from hydrogen-containing gases. An INDEPENDENT CLAIM is also included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a graph illustrating a relationship between a ratio of current density and cell voltage.    Operating point (b-1)    Lines of air mass flows of cathode gas (B-1, B-3)