A power source combination circuit has a combination circuit and a controller. The combination circuit includes a first power source, a first field-effect transistor, a second power source, and a second field-effect transistor. The controller is configured to control the combination circuit to supply power to a load circuit, obtain a first input voltage, a second input voltage, and a third input voltage, and diagnose, based on the obtained voltages, whether the combination circuit is in a normal state or an abnormal state. The first input voltage is an input side voltage of the first field-effect transistor, the second input voltage is an input side voltage of the second field-effect transistor, and the third input voltage is a voltage at a power source input end of the load circuit.