Repeating POST test, then shutdown

[dld@law.pace.edu]

This is a bit of a long story, so please bear with me.  I have a Z170X-UD3 board with 64Gb Kingston HyperX memory and a GT-750i graphics care.  The processor is an Intel Core i7.  I do not recall the chipset and at this point cannot recover it from the machine.  I run Kubuntu 16.04 (a Linux flavor) with a Windows 10 VM.

A few days ago, the machine slowed down dramatically.  Checking Task Manager on the VM showed that the C Drive was running at or near 100% read, but I was not able to determine what it was trying to read.  Without the VM running, the Linux box operated at normal speed.  Whenever I see a slowdown, my first thought is to check the CPU temperature, but it was operating at a consistent 40C.

Then yesterday, I noticed that the mobo was showing a constant AO code, which means it was querying IDE drives.  But there are no IDE drives; the whole machine is SATA. 

So I thought the HD might be failing (although that should have affected the Linux speed also), and I installed a new Seagate 2TB drive and recreated both the Linux installation and the Windows 10 VM. 

Nothing changed . . . until today.  Now the machine won't boot at all.  It cycles through part of the POST test (C1, C2, C4, C5) two or three times and then shuts down.  I have checked for shorts but cannot find any.  There is also a jagged yellow light showing up on the board under the openings for additional cards; I've never seen anything like it before.  I have another machine with the same mobo and the same OS setup; it's doing fine.

I think the board is trying (in its own subtle way) to tell me that it is dying or dead, but I don't know what to make of this weird series of events.  Thanks in advance for whatever suggestions/advice you may be able to offer.

[shadowsports]

Hi Don,
Boards equipped with LED indicators can assist you with diagnosing boot problems.  They are different from the Debug LED display located at the top right corner of your board.  I was not aware this model had them.  Look closely and tell us which is lit? (CPU, VGA, etc) The PCB will be stamped.

Have you tried resetting your CMOS?  Please disconnect the 24-pin power connector from the board.  Now use a jumper or flat blade screwdriver to short the CLR_CMOS pins located at the lower right corner of your board.  10 secs should suffice.  Reconnect power and attempt to power on the system, if successful, press <DEL> to enter BIOS and make whatever changes are necessary for the system to boot.

If the above does not help, you will need to perform further troubleshooting.  Remove GPU, all but one stick of RAM, any add on cards, hard drives, etc, until the system will start, give beep error codes or exhibit other behavior.

If you have made changes to your hardware, BIOS, or updated the BIOS and this behavior has resulted, please share this with us.

Is your CPU fan spinning and any other observations?

[dld@law.pace.edu]

Shadow,
Thanks for getting back to me so quickly.  When resetting the CMOS earlier, I had not disconnected the power connector.  I tried it your way, but no joy. 
In answer to your question, there are no LED indicators lit during the abortive boot process.
I shall begin the further troubleshooting that you describe and report back.
I did try flashing the BIOS, but after this problem surfaced.  I also replaced the hard drive at that time.
Yes, the CPU fan is spinning as it should.

Is it at all conceivable that this is a power-supply problem?  It shouldn't be; the power supply is less than a year old and is of good quality.  I suppose if all else fails I can always swap it out and see whether that makes a difference, but that takes down the machine I'm using now.

In any case, here's a picture of the part of the board showing the yellow jagged line.  Perhaps it was there all along, and I just never noticed before, but I don't think so.

[dld@law.pace.edu]

   I followed all the suggested steps, beginning with a single memory stick and no hard drive or other peripherals.  It booted.  Then I reinserted the other sticks, one by one and rebooted successfully every time.  Then I reinstalled the GPU and still was able to reboot.  Finally I reinstalled the hard drive, and still could reboot.  Go figure!  My best guess is that some connector was not fully seated but not so poorly seated as to be visible.  Perhaps I should have known.  Lots of my brain's connectors are not fully seated either, but I've been unable to find a fix.  Every morning I reboot myself, but it never seems to help.

I am very grateful for Shadow's help.  It put me properly on the step-by-step path. 

Now I'm back to the problem that began this whole thing:  Windows 10 trying to read the hard drive (even the new one) 100% of the time.  But that problem is clearly not unique to this machine; the internet is loaded with stuff about that and suggested approaches.  I really doubt that it's a Gigabyte issue.

Profound thanks.Update:

[shadowsports]

Greetings,
The yellow LED is called path lighting.  Its a visual feature that delineates the isolation of the audio components from other electrical parts on the board.