Question Reliable PC suddenly failing to POST. MB standby LED on but no response. Dead MB or PSU or something else?

Apr 28, 2021
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Hi all,

Apologies if someone finds a duplicate for this thread - any similar ones I've found, the solution hasn't worked for me, so I figure I'll ask my own question.

I stepped away from my PC to take lunch yesterday, and it had switched off by the time I returned. No response at all when I press the power button. Bad switch maybe? Also no response when I jump the power switch pins myself.

I've stripped it right down to core components (it needed a good dusting anyway) to help diagnose the problem:

  • ASUS Z97-PRO GAMER motherboard
  • Intel i5 4690K CPU
  • 1x4GB HyperX 1866 DDR3 RAM in A2 slot (originally 4x4GB)
  • EVGA Supernova NEX650G modular ATX PSU - 5 yrs old but no issues til now
  • Samsung 850 EVO SSD 250GB (boot drive)
Also connected:
  • Corsair H55 AIO cooler (fan and pump connected to CPU_FAN and CPU_OPT on mobo)
  • CPU power, MB power, and SATA data & power cables
  • USB and HDMI out to keyboard and monitor
That's absolutely everything connected in the system right now, no luck with posting let alone booting.

When I power on the PSU, the motherboard's SB_PWR (standby power) LED comes on. No other response.

When I jump the power_sw pins, there's a small ramp on the PSU fan, but no other response.

When I disconnect the MB cable from the MB and bridge the GND and PS_ON pins, there's again a small ramp on the PSU fan and a slight charging of the capacitors (any other pwr/gnd voltage diff gives a steadily declining voltage after the PS_ON is grounded). PS_ON and +5VSB* are steadily at 5.2V whereas no other MB cable pins have a voltage.

I've not had a component outright die on me yet since I built this in December 2015 so I've not had to diagnose the error before.

Does anyone have any advice about what might be failing and/or what tests might help verify this?

Thanks a bunch,
Lexi

Follow up: I've removed the RAM and SSD. None of the Q LEDs on the board light up (particularly looking at the DRAM_LED). This has to be the PSU or motherboard right?

* correction
 
Last edited:

avg9956

Commendable
Apr 7, 2019
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I would not eliminate the possibility of the motherboard, see if you can spot any burnt or inflated capacitors.
However at this stage, its easier if you can get a hands on another PSU to test it out. Trip to the computer repair store would do.

When I disconnect the MB cable from the MB and bridge the GND and PS_ON pins, there's again a small ramp on the PSU fan and a slight charging of the capacitors (any other pwr/gnd voltage diff gives a steadily declining voltage after the PS_ON is grounded). PS_ON and +5VSB* are steadily at 5.2V whereas no other MB cable pins have a voltage.

None of the Q LEDs on the board light up (particularly looking at the DRAM_LED).
Judging by your description that the PSU at least still runs.
2015 and its now 2021, a good 6 years. There may be some visible wear on the caps on the motherboard. Usually motherboards are durable, but that also depends on what capacitors they use and their VRMs.

I stepped away from my PC to take lunch yesterday, and it had switched off by the time I returned.
Although I really don't suspect that for like the vast majority of the cases and it is highly unlikely, but if you happen to have a screw or any metallic object lying around inside the case and happen to short something then that would possibly explain it.
 

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