[SOLVED] ASUS Z97-A BOOT_DEVICE_LED always on at startup

[dizcza]

I have

• ASUS Z97-A motherboard
• Chieftec 750W silver (repaired twice)
• Two SSD drives
• Intel(R) Core(TM) i5-6267U CPU @ 2.90GHz
• NVIDIA GTX 980 connected to a display via HDMI

and 99% of the time at PC bootup I see a black display (it's powered on and becomes a bit lighter when I turn on the PC) and BOOT_DEVICE_LED is always ON. The rest of motherboard status LEDs are off. I even don't see the ASUS logo. Just a blank screen slightly lighter than in the powered-off state. And once in a while, after I reboot the PC with the power button one hundred times, the BOOT_DEVICE_LED is off and I see the ASUS logo and the PC is booting correctly with no errors until I power it off and on again. Subsequent soft restarts (programmatically) are OK once it succeeds to boot up in the first place.

Sometimes I see the ASUS logo and even the dual boot-up screen (I set up Linux and Windows) and then everything freezes followed by a restart (not always; it could freeze and stay).

The Chieftec PSU has been repaired recently - it provided 10.90 V instead of 12.0 V, but now it's a steady 12.33 V. Other values - 3 & 5 V - are also fine. (When it was 10.90 V, the CPU didn't get powered on.) After it's been repaired recently and the BIOS reset with CMOS removal, the situation got more severe: now only 1-5% of times the booting succeeds, before - 10-20%. A multimeter won't show power supply peaks and valleys, it's hard to imagine that the problem was and still is in the PSU.

The problem is that I have no idea where the issue might be. I suppose it has something to do with the motherboard, but I'm not sure. I also don't know how to investigate the behavior thoroughly.

Before I contact a local technical service shop, I give it a try to post the problem here and at least narrow down the search space if not solve the problem completely.

What I've already tried:

• Plug out and plug in (after a few minutes) the CMOS battery. It did the BIOS reset though.
• Clear RTC RAM as suggested in https://www.manualslib.com/manual/1151568/Asus-Z97-A-Usb-3-1.html?page=33
• Plug in and out the SSD, left only one, tried different SSDs and ports on the motherboard.
• Plug in and out RAM sticks.
• HDMI and DVI display interfaces.
• I checked the motherboard for a visible short circuit and found none. I have a multimeter, don't know how to measure shorts properly, and I'm a bit afraid to actually short the board accidentally with the multimeter leads - I've already fried a cheap ESP32 board poking around with multimeter probes.

I've been trying to boot it up for half an hour and didn't manage it. So I cannot even run some disk checks commands (though I doubt the problem is in SSDs - I've tested two old and one new).

 

[Lutfij]

Chieftec 750W silver (repaired twice)
You're causing more problems with this...

 

[dizcza]

Turned out to be an error in BIOS. After it's reflashed with an external programmer, the issue is gone.
Nothing wrong with the PWU.