Fault Finding is an important part of DIY work.
There is more than one way to go about fault finding.
Divide & Conquer
This is the most powerful means to find a fault, and is normally the method to use for complex systems.
The idea is to narrow the fault down by seeing if its in one area or another of the system, and repeatedly narrowing it down to less and less possible parts, usually down to the one faulty part.
The bad part can then be repaired or replaced.
The problem with this method is it requires some ability to fault find!
Guess & Replace
A popular method for people with little idea where to begin. A good way to waste money.
Success is likely with very simple systems, eg with 2 or 3 parts. Success is very unlikely with complex systems eg electronic goods, which contain 100s of parts.
Replace the lot
A much over-used method.
A fair strategy when faulty part replacement or repair are not likely to be possible, or when the cost of fault finding is expected to exceed the cost of total replacement.
A great deal of money is wasted every year by use of this method.