Question Video output issues with used HP Envy ?

[elmoemmett]

I got a used HP Envy off of Marketplace. Here are the specs that I have:
i7-3770K
GeForce GTX 1060
No OS installed yet
I'm not sure how to get the motherboard or PSU model info, since this is a prebuilt machine. I can say that the psu is not modular.

It worked when the seller showed it to me, although it was a bit finicky getting it to output to the display. I thought in the moment it was a cable issue. Got it home, and it's got issues. When I try to boot it, I get 6 beeps. All of the fans turn on*, and the machine seems to boot up, but I get no video output. Based on my research, this means motherboard, gpu, or video card failure. Here is what I've tried so far:

1. Removed the GPU and plugged directly into the on-board video output. There are no beeps and the machine seems to boot up properly, but I get no video. I'm using a DVI to HDMI cable that I know works. One thing to note is that both DVI ports on the machine were covered up with plastic pieces fastened with hex screws. This makes me wonder if HP built something into this model to make these outputs unusable?
2. Checked all cables to ensure things are connected. Everything appears to be in place.
3. Did a battery reset. Made no difference. I tested the battery, and it's in great shape.
4. Tested the GPU in another machine, and it's working as expected.

*One more thing to note. When the GPU is in the HP machine and I get 6 beeps, all of the fans turn on EXCEPT the GPU fans. They run intermittently.

Really not sure where to go from here. Help is appreciated. I'm about ready to chalk it up to motherboard failure and salvage what I can, but thought I'd post here just to see if anyone has any ideas.