Taking up from the previous post I want to add one more thing.
For almost every concern or symptom in the vehicle there is a list of things that possibly could lead to this concern or symptom. The more you know about cars – the longer the list goes. What you think about if you see flat tire? Puncture, is it? Well, most likely, but not the only possible reason. How about leaking, broken or missing tire valve? Some rims have two of them by the way so if you see one that is intact and OK you still might have a problem with another one, ha-ha! Cracks or pores in alloy wheels or leaking welding joints in steel ones are also not unknown (although rare). Add to the list bent rims from hitting a potholes. And then someone might had your tire cut with a knife or even shot with a gun. See, there is quite a list already and this is a simplest concern I could have imagined.
So when your vehicle gets some more complicated concern such as engine light comes on, no start, loss of power or heating stops working – the list of possibilities could literally have hundreds of entries.
Therefore diagnostics is a work performed with the intent to reduce this list, ideally down to a single entry. You can do it by replacing parts one by one until you catch the bad one – this is one of the ways to do diagnostics although it gets expensive quickly if you are not lucky enough to get successful fix from the first try. Spending time on performing series of diagnostic tests is another way of doing that and in most cases it is a cheaper way.