A method for diagnosing operating condition of a device for cutting off and connecting a high-voltage bipolar battery from/to a bipolar onboard power network of a motor vehicle. The device includes a first power relay between a first terminal of the battery and a first terminal of the network, a second power relay between a second terminal of the battery and a second terminal of the network, and a serial arrangement of a third precharge relay and a precharge resistor, connected in parallel to the first power relay. The method: measures two voltages of the cutoff/connection device; compares the measured voltages with predetermined voltage values representing faults of the cutoff/connection device; and diagnoses a fault of a relay used on the result of the comparison.