Among the PGNs required for all RV-C nodes is the Diagnostic Message. This PGN includes a general status indicator (red/yellow/green light), and if there are any problems with the node, a list of codes identifying the errors. Each error is identified with a System Identifier (“SID”) and a Failure Mode.

The SID is taken from a list of components defined for that particular product. For example, the generator start relay is assigned a particular SID. The Failure Mode identifies the way that particular component failed - “mechanical failure”, for example. By using SIDs and Failure Modes, all errors are distilled to a set of numbers transmitted on the network.

A simple diagnostic tool needs only to ask each node for its diagnostic status, and translate the SIDs and Failure Modes into terms a user can understand. Of course, many nodes will have more sophisticated tests implemented in their software.