A method for functionally checking a vacuum switch of a traction inverter having a grid-side four-quadrant chopper and a load-side pulse inverter electrically connected in parallel on the DC side by a DC intermediate circuit. The AC side of the chopper is connected to a secondary winding of a traction transformer, with the primary winding of the traction transformer connectable to an AC grid voltage by a vacuum switch. When the vacuum switch is open, the chopper is actuated precisely when the AC grid voltage is temporally located relative to the chopper input voltage, such that the amplitude of a voltage difference between the AC grid voltage and the chopper input voltage corresponds to a predetermined test voltage. It is then checked whether current flows from the AC supply grid to the chopper. The functionality of the vacuum switch can thus be checked at any time without a test device.