Abstract: A fault diagnosis method includes detecting a state of a relay device, sending a fault diagnosis trigger signal to the relay device based on the stateto enable the relay device to enter a fault diagnosis mode, where the fault diagnosis trigger signal includes one or more of a variable supply voltage value, a transmit/receive control signal, and a diagnosis control signal, detecting a state of the relay device in the fault diagnosis mode, determining a fault code based on the state, and determining a fault based on the fault code.