First -- wrong category of forum for this thread. This should have been posted in the mining section (mods will move it I am sure)...

That being said, like the others have pointed out -- never just install all 6 GPUs and turn on the rig. This is a sure way to fail to boot. Also I am not sure if the Z170a SLI Plus can run 6 GPUs (a quick Google search said it was limited to 4 -- this could be outdated however). I have several rigs running the Z170A Gaming M5 board and 6 GPUs work fine.

Install one GPU and the bare minimum of hardware and boot. Once you have successfully done that, install the remainder of your hardware (RAM, ROMs, Extra HDs, etc) and turn the rig off and add another card. Repeat this process until you get to the 3rd card.

Before you install the 3rd GPU, make sure you have your BIOS settings correct:
-- change PEG0 and PEG1 to Gen1 (some of my rigs want Gen2 -- most are fine with Gen1)
-- set Above 4G Decoding to Enabled
Save settings and reload.

Shut down -- Install the third card. Boot and allow drivers to install. Again repeat the process of adding one card at a time and shutting down until all 6 GPUs are installed.

If you get to a point where you receive a failure to boot or drivers won't install, it's probably the riser in the newly added card. The risers have HORRIBLE quality control, especially with the v 1.1 and 002 boards. I have had about half of my Version 2 risers fail. Look for risers with Version 6 or above boards. They are much less likely to fail.

Report back if this doesn't help you and I am sure we can throw some more ideas at you!