Question PC won't boot unless 24-pin PSU plug unplugged and re-plugged after shutdown

[mxnerd]

It's been 4 days.

Motherboard or PSU problem?

Unplug and replug the power cable does not help.

 

[VirtualLarry]

Could be either mobo or PSU, really.

I would, in this order:
1) Replace CMOS battery (while unplugged), then do a CMOS_CLR, then plug in, and attempt to power-on.
2) Try swapping a known-good PSU into the system.
3) Try swapping mobos.

But yeah, that's a wierd problem, but I have a hunch that the problem has something to do with the +5VSB of the PSU, and the mobo's power-on circuits that depend on that. CMOS battery could also be involved.

Also, check how many USB devices you have plugged in, and if possible, unplug them all except for keyboard + mouse.

Also, if using DisplayPort, change DP cable for one that is VESA-compliant (doesn't pass power on pin 19). (Bad / non-compliant DP cables can affect +5V line in PC, which may be tied to +5VSB line if USB ports are set to provide power to charge devices while PC's power is in ATX soft-off state.)

I've had a couple of buddy's rigs, that stopped immediately powering-on, there was a "delay" of seconds to minutes powering on from pressing the front-panel power switch. Sometimes, unplugging or switching off the PSU in the back and waiting and switching it back on would allow the PC to power-on. In both of those cases, replacing the PSU (*weak / bad +5VSB from PSU) fixed the issue.

But you mentioned trying the unplugging trick.

 

[Iron Woode]

Last time I had problems like that my PSU was on it's last legs.

 

[Furious_Styles]

More likely PSU than mobo.

 

[Steltek]

What are the model numbers of your system components (motherboard/psu/gpu)?

 

[mxnerd]

ECS Z97-PK, Pentium.

Replaced the CMOS battery, seems working! Will see.

 

[UsandThem]

ECS Z97-PK, Pentium.

Replaced the CMOS battery, seems working! Will see.

It's not very often, but I've seen a few users over the years get their PC working by just replacing a dead CMOS battery. In most cases, a dead battery would just cause a motherboard to lose its settings when unplugged from wall power.

It's odd for sure, but if it works it works.

 

[mxnerd]

In most cases, a dead battery would just cause a motherboard to lose its settings when unplugged from wall power.

Yeah, that's the usual experience. This one is really weird.

 

[VirtualLarry]

I know that it seems a bit like "PC Tech Voodoo", but one of the first things I try, if an (older, previously-working) system isn't booting / POSTing / powering-on any more, is try to replace the CMOS battery, if the mobo is a few years old.

I don't know all of the specifics, but on some mobos, it's not just for holding settings, it's actually utilized somehow in the low-level power-on process. Hence the reason why it matters / why it works, "for some boards".

Glad that I could be of some (slight) assistance here.

Edit: That doesn't (necessarily) mean that your PSU doesn't have a weak +5VSB as well, as that powers the CMOS when the PSU is plugged into the wall.

Maybe:
1) CMOS is necessary for boot process -> bit set in CMOS to determine warm/cold boot on BIOS reset
2) CMOS powered by +5VSB when PSU plugged in
3) CMOS powered by battery when not plugged in / weak PSU +5VSB

So, maybe the +5VSB on your PSU is shot, which caused the battery to drain, causing the system not to boot.

Maybe.

 

[mxnerd]

Edit: That doesn't (necessarily) mean that your PSU doesn't have a weak +5VSB as well, as that powers the CMOS when the PSU is plugged into the wall.

Maybe:
1) CMOS is necessary for boot process -> bit set in CMOS to determine warm/cold boot on BIOS reset
2) CMOS powered by +5VSB when PSU plugged in
3) CMOS powered by battery when not plugged in / weak PSU +5VSB

So, maybe the +5VSB on your PSU is shot, which caused the battery to drain, causing the system not to boot.

Not familiar with PSU, so never heard +5VSB before.

You could be absolutely correct on this +5VBS thing, however.

The PC experienced the same booting problem once in a while in the past, but this time it just won't boot anymore every time after shutting down.

 

[Steltek]

Not familiar with PSU, so never heard +5VSB before.

You could be absolutely correct on this +5VBS thing, however.

The PC experienced the same booting problem once in a while in the past, but this time it just won't boot anymore every time after shutting down.

ATX PSUs produce the following standard voltages (called rails): +3.3v DC, +5v DC, -5v DC (may not be used, not required on ATX1.3 or later PSUs), +5v SB, +12v DC, -12v DC.

@VirtualLarry is suggesting that the +5v SB rail of your PSU may be weak or failing. The SB refers to "Stand-By" power -- the +5v SB rail is always active except when the computer is physically disconnected from the power plug (either unplugged from the power recepticle, or if the PSU has a separate power switch on it that is flipped off). It, among other things, allows the computer to wake from sleep (wake on LAN, wake on ring, etc) and maintains the CMOS settings to minimize the drain on the CMOS battery. A weak or failing +5v SB rail at minimum will drain the CMOS battery, causing the system to loose its BIOS settings.

BTW, if the machine doesn't continue to work and you have to try to replace the PSU, just set the new PSU up outside the case and simply swap all the motherboard and gpu power cables over to try it out. That way, you haven't wasted the time installing it only to find out it wasn't the problem.