Member Avatar for boogatoo

A.k.a. The computer won't "turn off."

I'm working on a buddy's computer because he's having an odd problem. The machine was off (though still plugged in) during a rather pyrotechnic lightning storm. The PC is plugged into an oldish surge protector, but it has worked just fine for many years. When he came home after the storms, he noticed that his PC had turned itself on, but the monitor did not show anything.

The issue is that as soon as the PSU receives power (by using the switch on the back of the PSU), the fans (Processor and Case), HD, CDROMs, GFX card, etc. immediately power up. The computer doesn't boot just yet, but it will if you press the front power button. OS boots up fine, everything seems to run well. If you shut down the OS, it goes through the normal shutdown procedure and looks like it turns off, but all the fans are still running (proc, case, gfx). I disconnected everything from the mobo and powered it back up, no change. Well, the mobo complains about not detecting any RAM, but that's normal.

I'm going to head over tonight to swap out the PSU with an extra one that I have, just to make sure whether it's the PSU or the mobo that's having issues. There was no scorching, no interesting smells, no funny sounds or anything else that would lead me to believe the PSU or mobo are bad, other than the behavior. I'm almost 90% convinced that the mobo has a perma-short somewhere, and can't seem to regulate the power coming from the PSU, but I'll find that out for sure tonight.

I'm posting this because I was unable to find another thread with this particular problem (though there are some similar ones) and I thought I would get some input from those who have had similar situations. Also I wanted this to be a record for others to see if they have a similar issue in the future. Anyone have any ideas or suggestions that differ from what I'm going to try? I also have a duplicate of the mobo (from my current PC) that I can swap out later to test, but I'd hate to fubar my own mobo just to find out that his PSU is where the problem lies. Anyway, thanks for reading and any help that could be provided.

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Dani AI

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Good troubleshooting notes from and a solid nudge from . A short technical add-on that fills gaps and gives a safe, repeatable debug path.

Modern ATX supplies always provide a small standby rail (5VSB) as soon as mains is present; the motherboard pulls the PS_ON line low to tell the PSU to enable the main +12/5/3.3V rails. The symptom “fans/drive spin as soon as the PSU gets mains and stay running after shutdown” points to either (a) the PSU’s control stage failing (main rails never switch off) or (b) the motherboard holding PS_ON low (short or damaged control circuit). Lightning or a surge coming in via a data line can damage either side, so both hardware and external surge paths should be considered.

Quick, practical checks (safety first — unplug before touching, use insulated tools, and don’t probe live mains if untrained):

  • Visually inspect the board and PSU for burn, bulging caps, or scorch marks.
  • With mains connected but the system “off,” confirm ~5V on the purple 5VSB wire (multimeter).
  • Disconnect the front-panel power header: if mains-on still powers main rails/fans, the PSU is likely at fault.
  • Measure the PS_ON (green) pin: pressing the front power button should pull it low to switch the PSU on; after shutdown it should return high. If it stays low, the board is holding it.
  • If a quick swap with a known-good PSU is available, that’s the fastest definitive test.

Prevention and follow-up: install proper grounding and a coax grounding block at the demarcation, use surge-rated protection (and an inverter/UPS with surge suppression), and document damage to pursue reimbursement if a provider’s negligence contributed. If a network port is dead, a replacement NIC is a low-cost fix; if multiple subsystems act oddly afterward, treat the motherboard as suspect and bench-test carefully or replace it.

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Member Avatar for sittas87

Hey there
It sure sounds like a PSU surge malfunctioning. you can swap out the PSU with a known good one

Member Avatar for boogatoo

Yup, that's what it was. I swapped the PSU and everything was fine. Seems like the PSU was just always giving full power to the motherboard, rather than the 5v low power rail while it was "off." Well, at least it was a quick and easy fix.

Member Avatar for sittas87

luckily for you

Member Avatar for boogatoo

Turns out that there was a lightning strike that caused this. It didn't come in through the power, but through the cable modem. The morons that installed his cable modem service didn't ground the cable line at all. It also fried the network part of the motherboard, but a $15 NIC made that right.

I tested everything I could think of with the computer and it seems to be functioning correctly now. My hope is that the cable company will reimburse him for the PSU and NIC. At least it won't happen again, as everything is grounded (twice, through a UPS).

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