In the medical arena, the definition of a diagnosis is pretty clear cut - you go to a trained professional and are told what ails you. In the automotive industry, the lines have been blurred. The availability of on-line advice and parts store willingness to scan a vehicle free of charge has changed the driving public's perception of the most important first step in the repair of a vehicle.
Auto repair facilities themselves subscribe to databases that aggregate repair information from shops worldwide. Those same shops belong to automotive forums, trade groups and have accumulated relationships within their communities all of which they use to inform their decisions about what is needed to repair your car. But the written or spoken word (to date) has never, by itself, repaired a vehicle.
All accurate information is helpful in the hands of a committed technician whose first priority has become the most expedient and economical repair of your vehicle. It is the notion that one piece of information whether acquired from an online source, water cooler talk, or a first-cousin once removed can be a magic bullet, which causes angina for technicians. Unfortunately, that "one piece" of data is just that and does not constitute a diagnosis much less a repair path.
A true diagnosis combines information from a number of sources including the owner's experiences and perceptions, measurable data of the vehicle's performance gleaned in real-time through the use of sophisticated diagnostic equipment and a look at the vehicle's Achilles Heels which is available through industry repair aggregators. But this only generates a theory, not a diagnosis not to mention a repair.
The diagnosis then needs to be verified. Technicians use a number of methods to confirm a suspected failure ranging from manipulating a component through the use of a tool to something as low-tech as a wiggle test. Remember, all this effort is going into a diagnosis- the first step. So, when you are certain that you know you have struck diagnostic gold on the internet, be reminded of that old saw, "When you're carrying a hammer everything looks like a nail."