I have two Optiplex 790 SFF systems, both out of warranty.

One system displays the yellow flashing power LED and does not power on when the button is pressed, which I know is due to a PSU failure and therefore I will replace that.

However the other system has been working fine until now whereby it started to complain there was no bootable device found, after which it would power off by itself before I could press F1 or F2, but now when I press the power button it comes on for a few seconds (POST lights flash on/off) but then powers off and does not come back on when the power button is pressed. The power button does not flash when pressed, it just does nothing. Disconnecting and the power cable and reconnecting it a minute later, there's no self-test, and pressing the power button does nothing. However, leave the power disconnected for at least 5 minutes and reconnect it, we get a very brief self-test, and the power button now responds, but the same behaviour repeats as in it powers off after a few seconds and the power button no longer responds.

I've seen the first issue numerous times but the second issue has me stumped. I don't know if it's a dodgy PSU or something else. There's no blown capacitors on either board and on the second machine both the incoming and CPU fans are spinning up, as is the power LED on the DVD drive, until it just dies.

Any ideas as to what could be the problem? Is it's a dodgy PSU, I'll order a second one, otherwise if it's anything else it might just be cheaper to buy a new system.

  • Can we get make and model of the motherboard that system is with? Or are they the defaults in the specs? This sounds more like a faulty motherboard rather than PSU to me.
    – Fanatique
    Jul 29, 2019 at 11:45
  • 1
    Disconnect all drives and peripherals, see if it will power up now, any faulty piece of hardware can cause this power issue.
    – Moab
    Jul 29, 2019 at 16:05
  • Standard specification, disconnected all peripherals so it's only the fans connected and the power cables, system powers on and emits one long beep followed by pause then two short beeps, then dies. Doesn't power on after that. Leave it 5 minutes and only at that point can one try again; system does the exact same thing.
    – Reado
    Jul 30, 2019 at 8:01

1 Answer 1


I put the PSU from the first system into the second and got a flashing amber power LED, so the system board must be working fine. To prove this point, I put the PSU from the second system into the first (the first being the one that originally had the flashing amber power LED) and the exact same thing occurs as it did with the other, as in the system powers on but part way through POST it just dies and won't power back on. I think it's safe to say the PSU in this one is also at fault, so I'll order one and see what happens.

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