Failure diagnosis device

A failure diagnosis device for a communication network system, wherein:

• the communication network system includes
• a plurality of nodes,
• a plurality of communication buses,
• a plurality of communication cables,
• a first network in which a relay device that relays CAN communication to which a CAN protocol is applied and a group of nodes are connected in parallel through the communication buses to perform the CAN communication, the relay device and the group of nodes being included in the plurality of nodes, and
• a second network in which a switching device that relays Ethernet communication to which an Ethernet protocol is applied and another group of nodes are connected to each other in a one-to-one manner by the communication cables to perform the Ethernet communication, the switching device and the other group of nodes being included in the plurality of nodes;
• the relay device and the switching device are connected to each other through the communication buses;
• the node included in the first network including both a target node and the switching device, the target node being the node that is connected to the relay device through the communication bus and is connected to the switching device through the communication cable, executes, when the Ethernet communication executed between the target node and the switching device is interrupted, reading of diagnosis information that is a record indicating that an abnormality is detected by a self-abnormality diagnosis function of the target node and the switching device by using the CAN communication;
• and
• the failure diagnosis device confirms a result of the reading of the diagnosis information, and determines, when the diagnosis information is recorded in any one of two nodes that are the target node and the switching device, that a failure has occurred in the node in which the diagnosis information is recorded, the node being one of the target node and the switching device.