PC won’t power on with CPU installed, but powers on without CPU.

Posted by Severe_Sweet_862@reddit | buildapc | View on Reddit | 9 comments

I’m trying to diagnose a weird failure and would appreciate some help walking through the logic.

Specs

CPU: Ryzen 5 5600X

Motherboard: MSI B550M PRO-VDH WIFI

PSU: Cooler Master MWE 650 Bronze

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Original failure

The PC was running normally when suddenly the display cut out. The system still had power. When I tried restarting, the CPU debug LED came on and the system wouldn’t POST.

After powering it off and waiting, the system became completely unresponsive — no POST, no fans, nothing.

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Troubleshooting steps so far

Removed everything from the case and tested on a table to rule out a short.
Paperclip test on the PSU → PSU fan spins.
Took the motherboard to a shop:

- They connected their PSU (24-pin + CPU 8-pin)

- The motherboard powered on with a CPU fan attached

Back home, I tested again with my PSU:

Configuration A (NO CPU installed):

- 24-pin connected

- 8-pin CPU cable connected

- PSU fan spins

- motherboard powers on

Configuration B (CPU installed):

- same cables

- press power

- absolutely nothing happens

- no PSU fan, no twitch, nothing

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Key observation

The system powers on without the CPU installed, but the moment the CPU is installed the system refuses to start.

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My thinking so far

The failure appears when the CPU power rail becomes active.

So the possible culprits seem to be:

PSU EPS rail failing under load
Motherboard VRM failure
CPU short/damage

But I’m not sure which is most likely.

---

Questions

Does this behavior usually indicate a failing PSU EPS rail?
Could a dead CPU cause the system to refuse to power on entirely?
Has anyone seen this specific behavior with a B550 board?

Any suggestions on what to test next would help a lot.

[-]

jili-guala@reddit

Hey I have the exact same issue — MSI B550M MoBo POSTs when no CPU is installed, but absolutely no power when CPU is mounted. Have you figured out a solve?

Additionally, 1/10 times the system will POST but VRAM LED lights up (I’ve tested RAM sticks in all 4 slots), then shut down altogether after a few seconds. Any leads would be greatly appreciated!

The mobo was the issue, I got it fixed from a local technician, but the entire internet says the mobo is dead. Take your system to a local shop so they can troubleshoot all the parts and isolate that mobo is the problem, then figure out from there. LMK if you have any other doubts man.

Appreciate your reply! Yeah mobo was issue for me too. I didn’t bother to find a shop cus it was new from Amazon, I had the old board returned and got a new ASUS B550 board and it turned on without an issue

deleted_by_reddit@reddit

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nova-bil@reddit

It seems a short circuit behavior. Since the board powers on without the CPU but stays dead with it, your motherboard's VRM or the CPU itself is likely shorted. When you drop the CPU in, the PSU’s OCP (Over Current Protection) kicks in instantly to prevent a fire, which is why nothing happens.

Since the shop got the board to power on (likely without a CPU or with a different one), your 5600X is the prime suspect.

Next steps:

-- Try your PSU on another system if possible to rule out the EPS rail.

-- Borrow a cheap AM4 chip (like an Athlon or 3000 series) to see if it posts.

-- Check the CPU socket for any burnt pins or debris.

RepresentativeIcy922@reddit

Well if you took the CPU and ram there together with the board, that kind of rules out anything but PSU failure doesn't it?