A network node (102 - 105) for coupling to a communication bus, the node comprising: a receiver configured to receive one or more messages from the communication bus from other nodes coupled to the communication bus; and a transmitter configured to transmit one or more first messages having a first message format on the communication bus for communicating with said other nodes coupled to the communication bus, the transmitter further configured to transmit one or more diagnosis messages having a second message format on the communication bus for use in determination of communication errors, wherein said transmitter is configured to send said one or more diagnosis messages having one or more of: (i) a predetermined pattern of symbols different to the symbols of the first messages; (ii) a predetermined sending schedule different to the sending schedule used for the first messages; (iii) a predetermined line encoding method different to the line encoding used for the first messages; (iv) a predetermined bit rate different to the bit rate used for the first messages; (v) a predetermined position in one or more of the first messages; (vi) a predetermined signalling frequency that is out of a frequency band used for transmission of the first messages; (vii) a predetermined signal strength different from the signal strength used to send the first messages. a receiver configured to receive one or more messages from the communication bus from other nodes coupled to the communication bus; and a transmitter configured to transmit one or more first messages having a first message format on the communication bus for communicating with said other nodes coupled to the communication bus, the transmitter further configured to transmit one or more diagnosis messages having a second message format on the communication bus for use in determination of communication errors, wherein said transmitter is configured to send said one or more diagnosis messages having one or more of: (i) a predetermined pattern of symbols different to the symbols of the first messages; (ii) a predetermined sending schedule different to the sending schedule used for the first messages; (iii) a predetermined line encoding method different to the line encoding used for the first messages; (iv) a predetermined bit rate different to the bit rate used for the first messages; (v) a predetermined position in one or more of the first messages; (vi) a predetermined signalling frequency that is out of a frequency band used for transmission of the first messages; (vii) a predetermined signal strength different from the signal strength used to send the first messages. (i) a predetermined pattern of symbols different to the symbols of the first messages; (ii) a predetermined sending schedule different to the sending schedule used for the first messages; (iii) a predetermined line encoding method different to the line encoding used for the first messages; (iv) a predetermined bit rate different to the bit rate used for the first messages; (v) a predetermined position in one or more of the first messages; (vi) a predetermined signalling frequency that is out of a frequency band used for transmission of the first messages; (vii) a predetermined signal strength different from the signal strength used to send the first messages.