I have worked as a service tech in the boating industry and before that, I sold audio & installed car electronics, so I had a god background in 12VDC devices, troubleshooting, etc. I did some component level repairs, but didn't need to, because we had a service department for that. Moving to servicing boats was a real eye-opener. The first day of technical training was all about 12VDC electricity and some of the devices that are used in a boat's electrical system WRT fuel injection. The Bosch-style relay is almost ubiquitous in cars, trucks, boats, motorcycles and other used- very simple, but somehow mysterious because it uses electricity. Some of the others at the session had been working on engines and boats for decades, but were completely dumbfounded by a device that has five metal tabs for the connections and a diagram on the side, to show which tabs are used for the latching coil, which are normally closed or normally connected. Couldn't even determine which did what, using a multi-meter.
Once I was in that business for a while, I started to see boats coming in that had problems which remained for quite a while, simply because the people who had looked at the boats couldn't find the source of the trouble. Might have been a bad wire terminal, loose wire, a short circuit, bad sensor, etc. These boats had been taken to dealers that had been in business for decades and still, they were unable to find a solution.
Fast forward a few years and I became involved in a ski boat user's forum and the first comment I felt compelled to reply to concerned the annoying warning buzzer that made noise as soon as the key was turned to ON or when the oil pressure fell, the engine overheated, or some other problem existed. The comment that the boat owner should cut the wire on a new boat is the reason I replied and that step could have caused a fire or death and at the very least, it would have voided the warranty which, because it's a warning device, would have been completly justified, IMO.
I replied in another thread about engine performance, after someone jumped in to mention that his engine had been hard to start when it was warmed up to normal temperature. I asked if the exhaust smelled of gasoline and if he had soot on the transom. He replied that he did, so I told him to check the coolant temperature sensor- he came back the next day, to say that the dealer hadn't been able to find the solution of the problem IN FIVE YEARS! I think that's pathetic. A dealer was charging him to service the boat and was unable to look in the service manual to check for stored codes, test the resistance, look at the exhaust ports or smell the exhaust.
Getting to the point of the end user servicing their own, the boat dealers/service shops who can't bring themselves to learn about what they're charging customers to do, people who don't have a tiny particle of knowledge about an engine or electronics are willing to jump head first into these problems, usually by replacing sensors that may have nothing to do with the problem, but that's what they know how to do- replace, rather than diagnose. They wouldn't think of doing this to their car because good car service shops are easier to find, even though the car may cost half of the price of the boat.
I think more should be done to force dealers to prove they can perform, or lose any factory authorization to service those products. If they want to charge for their "services", they need to justify their existence. I also think more independent service shops should be authorized by manufacturers when the number of dealer/servicers is limited and sparsely-distributed throughout a country. If someone can prove they know how, they should gain approval. These end users seem to forget that mixing electronics and flammable liquids can be deadly.