Wow! 62 years experience, looking to buy a two wheeler, and eager to learn the technology required to work on the bike. Congrats, Davetac! I'm impressed! I hope I can maintain my physical and mental condition to achieve that at what I roughly calculate your age to be.
As far as I know,(and I'm still a newbie at my Eldorado's workings, so don't consider me an expert) there is no way to use the speedometer readout to read trouble codes on this bike. It'd be a great idea for do-it-yourselfers, but manufacturers seem to want to make it difficult enough to keep their professional technicians busy.
There are ways to access trouble codes with software and adapter cables, using a laptop computer. Kiwi Dave was a big help with information on this. I've toyed with this on my bike, but thankfully, have not had to use it in regards to any actual problem. I'm approaching 12000 trouble free miles. Even some of the quirks of the fuel injection I've read about in these discussions do not concern me as anything to worry about. I was renting Harleys, Indians, and Victorys for several years before buying the Eldorado, and none of them seemed to be totally flawless, but fuel injection seems to be much better than carburetors. I kind of like having a less than perfectly smooth ride, gives the impression of a bit of brutality to the beast. Rough idle, no problem. Harley seems to even encourage that, as I am sure their market-share will plummet the day they make their engines smooth and refined.
If you do get the bike, feel free to message me for a walk-through on procedures like this. I'm sure it'll help me with my skills, as much as assissting you. I've been a professional auto mechanic for nearly 40 years, dabbled with working on my own bikes, and enjoy learning about the system on the Guzzi.