Diagnosing means identifying and isolating the cause of problem. Once you've identified the problem, you then proceed to troubleshoot. Troubleshooting is the attempt to fix the problem. Effective diagnosing and troubleshooting allows you to focus on fixing the actual cause of the problem and not just its symptom. Specially, when we talk about replacing the components and peripherals, you don't just buy and replace them unless you're pretty sure that's the cause the problem; otherwise it's just a waste of time and money.