It sounds as if you have figured out it's either the PSU, motherboard or CPU but want some test to figure out which.
Sadly the CPU test is to put the CPU into another working system.
The PSU test, most of the time is to use a Volt Meter and power up the PSU using the GREEN WIRE METHOD then check the voltages. HOWEVER this is a partial test since it can't tell you if the PSU works under load. For that we swap in another PSU.
Now that we have tested 2 of the 3 parts we can, at this point guess if it is or is not the motherboard.
--> Worth noting at 5 years is the BAD CAPS issue. Google BAD CAPS so you know what to look for.
I had the same PC for over 5 years. It stopped working while I was using it.
I started taking everything out and started with just PSU MB and CPU only. Here is where im confused... Motherboard will boot and stay on when I unplug the 4pin 12v connector.
So i took out the CPU and here is what happens...
* PC boots and stays on when CPU is out 4pin connector is in
** PC starts for a quick sec. but then goes off when CPU is in & or 4pin connector is in.
Whats causing my pc to do this?