The present disclosure relates to a Controller Area Network (CAN) system including: a CAN device and a monitoring device. The CAN device includes a transmit data (TXD) interface, a transmitter, a CAN BUS interface, and a control unit. The control unit reads out an identifier from a TXD message and compares the identifier with a reference tag. The CAN device generates a CAN BUS signal based on the TXD message at the CAN BUS interface. The control unit, if the comparison indicates that the identifier does not correspond to the reference tag, invalidates a representation of the TXD message by the CAN BUS signal and temporarily prevents another CAN BUS signal from being generated by the CAN device at the CAN BUS interface. The monitoring device receives an instruction message over a CAN BUS network and, in response, tests for reachability other CAN devices on the CAN BUS network.