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HP Z640 suddenly doesn't boot, fans on high no error codes

HP Recommended
Microsoft Windows 10 (64-bit)

I've had my Z640 for about 2 years now, and it has worked great.  recently it's been powered down for about a month, and when I powered it on (no changes), instead of a quick POST and looking at a Windows 10 login screen, I get the fans running loudly, specifically the desktop fans on low and the GPU fan on 100%, after about 3 minutes the desktop fans also spin up to 100% and nothing else happens.  I've let it continue for up to 20 minutes with no change. 



HP Z640 with 2nd cpu riser board, 2x Xeon E5-2650 v3, 8x 16GB Samsung DDR4 2133 ECC RAM, 1x 8GB NVidia Quadro K5200, various SSD, SAS, and SATA storage


What I've tried: 

boot up without video card - get 6 beeps and red flashes from the power button as it should, does not POST, just repeats 6 beeps 5x then repeats 6 red flashes on the power button

boot up with the video card - just get fans on low then high

new motherboard - no change

new power supply - no change 

new K5200 video card - no change

reset cmos - no change

set the emergency recovery jumper with and without a FAT32 USB drive containing the latest z640 BIOS BIN file

pulled all components and boot with just the main board, 1x cpu, 1x stick of RAM - 6 beeps no video card

add the video card - fans on high for the video card, low for the case, a few minutes later all fans on high

switch the emergency recovery jumper from pins 2+3 to 1+2, add the USB drive to a USB 2.0 port.   It gives the expected 8 beeps at power on, the usb drive briefly lights up, and then the usual gpu fans on high, case fans on low, minutes later all fans on high.  have let it run this way for an hour, but no further activity from the USB drive and no expected power down when BIOS update is complete.  


I've seen a few threads with Z 600-840 workstation advice for this, but never a fix.  hoping someone from one of those threads happen to see this and advise what finally worked.  At this point I've bought a 2nd Z640 from ebay that will be here in a few days, and will swap components around to try and identify the problem, hopefully ending up with 2 working z640s and not 2 junk pile money pits.  

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