Abstract: A diagnostic processor (24) is installed in a router (2) on the user side of a network termination. This processor can be downloaded from the network and can in turn be downloaded to user terminals in a local network. On detection of a fault, the processor retrieves data from a store of fault rectification instructions, for display in a form appropriate to a user terminal, giving the user instruction appropriate to the fault condition detected and the type of router being used. This allows the user to rectify the problem without recourse to online or telephone helplines, which may be unavailable because of the prevailing fault condition.