Question PC won't even power on.

[Vybz]

My PC is alittle dated and is now not even powering on. After running through quite a few troubleshooting steps I narrowed down the issue to the 4x4 pin power cable for the CPU. If I unplug it the system powers on but once plugged in it does not come on. It comes on with everything else plugged in.

I have tried to take out the CPU clean it and reinstall it with new thermal paste, but that has not worked. What else can I try? Is the issue the CPU? I was a Intel Core i5-8400.

 

[Lutfij]

We're going to need a little more information from your end. When posting a thread of troubleshooting nature, it's customary to include your full system's specs. Please list them like so:
CPU:
Motherboard:
Ram:
SSD/HDD:
GPU:
PSU:
Chassis:
OS:
Monitor:

If I unplug it the system powers on but once plugged in it does not come on. It comes on with everything else plugged in.
Can you elaborate on that line? You referring to needing to disconnect from the wall and reconnect to the wall outlet in order to get power to your system?

 

[Vybz]

Sorry here are my parts

CPU: Intel Core i5-8400
Motherboard: ASRock Z87 Extreme3
Ram: Team DDR3 1600 16GB
SSD/HDD: Crucal M500 240GB, WD Blue 1TB
GPU: MSI GAMING GeForce GTX 1060 6GB GDRR5
PSU: Seasonic FOCUS Plus Series SSR-650FX 650W 80+ Gold
Chassis: Corsair Obsidian 550D
OS: Windows
Monitor: dual dell monitors

 

[Vybz]

so any thoughts guys? should I buy a cheep CPU? cause I won't be using it for gaming any more. I don't think its the motherboard. what can I do? is the a way to test the CPU itself?