Diagnostic method for a bidirectional DC-to-DC voltage converter that comprises a first capacitor and a first full bridge at a first DC voltage terminal and that comprises a second full bridge at a second DC voltage terminal, wherein the first full bridge comprises two half bridges (11, 12) each having a first switching element (M1, M2) and a second switching element (M3, M4), wherein the first DC voltage terminal is decoupled from other electrical consumers, wherein the DC-to-DC voltage converter comprises a transformer that is electrically coupled to the first full bridge at a primary side and is electrically coupled to the second full bridge at a secondary side, comprising the steps of: providing a first DC voltage at the second DC voltage terminal of the DC-to-DC voltage converter; opening the first switching elements (M1, M2) and the second switching elements (M3, M4) in the first full bridge; charging the first capacitor to a predetermined first test voltage by driving the second full bridge; closing a first switching element (M1, M2) in the first full bridge; detecting a faulty second switching element (M3, M4) if the electric voltage across the first capacitor drops by more than a predefined first voltage value within a predetermined first time interval following the closure of the first switching element (M1, M2); opening the closed first switching element (M1, M2) in the first full bridge; closing a second switching element (M3, M4) in the first full bridge; and detecting a faulty first switching element (M1, M2) if the electric voltage across the first capacitor drops by more than a predefined second voltage value within a predetermined second time interval following the closure of the second switching element (M3, M4), characterized in that the second full bridge, in order to charge the capacitor, is driven such that an AC voltage is provided at the secondary side of the transformer and that an AC voltage thereby induced at the primary side of the transformer across the diodes arranged in parallel with the switching elements (M1... M4) of the first full bridge is rectified and charges the first capacitor. providing a first DC voltage at the second DC voltage terminal of the DC-to-DC voltage converter; opening the first switching elements (M1, M2) and the second switching elements (M3, M4) in the first full bridge; charging the first capacitor to a predetermined first test voltage by driving the second full bridge; closing a first switching element (M1, M2) in the first full bridge; detecting a faulty second switching element (M3, M4) if the electric voltage across the first capacitor drops by more than a predefined first voltage value within a predetermined first time interval following the closure of the first switching element (M1, M2); opening the closed first switching element (M1, M2) in the first full bridge; closing a second switching element (M3, M4) in the first full bridge; and detecting a faulty first switching element (M1, M2) if the electric voltage across the first capacitor drops by more than a predefined second voltage value within a predetermined second time interval following the closure of the second switching element (M3, M4), characterized in that the second full bridge, in order to charge the capacitor, is driven such that an AC voltage is provided at the secondary side of the transformer and that an AC voltage thereby induced at the primary side of the transformer across the diodes arranged in parallel with the switching elements (M1... M4) of the first full bridge is rectified and charges the first capacitor.