Some years back I used to work on some large machinery that utilised a Graphic User Interface...(-: Doncha just love the way manufacturers like to use their own language to make kit seem special...? The GUI connected to modules spread out over the length of the machinery and communication on a token ring network, which I suspected was not a lot different from can bus comms... Faults were caused by module failures, open circuits in the wiring, occasionally caused by poor connections points or shots in the wiring all of which could be diagnosed with a voltmeter, and as you suggest john, by unplugging modules individually and checking for the effect of doing so.. The diagnostic tool to use was a digital voltmeter. All of which had me go off in search of some info on can bus, which seems to be very similar system when it comes to fault finding..
I found this YT vid which kinda confirmed my thinking and might hopefully help..
