A network node for coupling to a communication bus, the node comprising:
a receiver configured to receive messages from the communication bus; and a transmitter configured to transmit first messages having a first message format and configured to transmit 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;
(ii) a predetermined sending schedule;
(iii) a predetermined line encoding method;
(iv) a predetermined bit rate;
(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; and
(vii) a predetermined signal strength different from the signal strength used to send the first messages.