Hello so I have a Dell Vostro 1520 and it turns on but the screen remains black, But an error code is shown so I looked up what it meant and the error code 'Flashing-On-On' means faulty system board and in the manual it says reseat cpu, replace mobo, replace cpu. So I reseat the CPU but it didn't work so, How do I know whether the CPU or motherboard is faulty or do I need to replace both or Do I need to replace one and if its one which one do I replace?
That flash code also means no processor detected. More than likely the motherboard. If you cannot even get to the Dell screen then you cannot run diagnostics. Normally replace both the CPU and the motherboard to be safe but your system is 10 years old and at that age anything else could give you trouble. Instead of putting any money into repairs, it may be time to consider replacing the system with a new and much faster laptop that you can enjoy for years to come.
I do plan on buying a better laptop but for the the time being I'm gonna use this for some light stuff. So as you said should I go ahead and replace the CPU without a replacement motherboard as those are costly!