Sep 30, 2019
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First time boot, all fans spin, motherboard RGB lights all come on, motherboard power light is on solid green. No POST beeps (but if I fully remove all RAM, I get beeps telling me theres no RAM). Red CPU light flashes and then goes off, Yellow RAM light flashes, stays on for about a second and goes off, no further LED's light up. Tried all the usual troubleshooting such as booting with one RAM stick tried it in each of the four slots, tried a different PSU still same outcome. 8 pin CPU 12v wire connected and motherboard 24 pin cable in fully.

Already tested different PSU, different RAM so my thoughts are it can only be motherboard or CPU

Specs are:
ASUS PRIME Z270-A
i7-6700K 6700K
Corsair CX750M
Corsair Vengeance LPX (2 x 8 GB) DDR4 2400
ASUS ROG STRIX RTX 2070
 
Sep 30, 2019
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If you have tried with the GPU out and everything else mentioned I would say motherboard. Only other thing you can try is pulling it out of the case and test it setup that way to make sure nothing is shorting it out.
Yeah I’ve already tried it outside of the case. It was a second hand board but it was sold as working but obviously I can’t vouch for that. I’ve seen some people mention that this board needs a BIOS update before using the 6700k but I can’t even reach the BIOS
 
Sep 30, 2019
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I don't think it needs a BIOS update. The Z270 chipset was released with Kaby Lake which is the 7th gen Intel CPUs. It should have had 6th gen support built in along with 7th gen support:

https://ark.intel.com/content/www/us/en/ark/products/98089/intel-z270-chipset.html

I would say the board is most likely faulty.
I’ve tried the new board, still doing the exact same thing. Going to have to try and find another CPU to test it with if that doesn’t work I’m out of options
 
Sep 30, 2019
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That is very odd. You might have a faulty CPU, although it is extremly rare. In my lifetime I have seen two. One out of the box (more rare than a unicorn) and one that got fried due to a lightning strike that fried every electric product in the guys home.
All signs are pointing towards a faulty CPU... I’ve borrowed a 6th Gen intel pentium and it works so updated the bios etc and then tried the 6th gen i7 and it didn’t come on