Diagnosis is something you have to learn. Most here don't have a clue. Not being mean by say it, just truthful. I'll rephrase that and say many are not knowledgeable. If you don't know how things work or can't figure it out, just throwing random parts at it hopefully will fix the problem. At the dealership or repair shop, you don't have the option to just throw parts at it. The customer won't stand for that.
Sounds like something I (try to) hammer into the heads of our (commercial HVAC) Service Techs: You have to be smarter than the equipment you're working on, and know how it works, before you can begin to figure out what's wrong with it.
Some of our guys "get it". The others, well...There's always The Bench at the Local.