Weird behavior of the first PCIE Slot on MoBo

Jul 24, 2018
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Greetings,

When I built my PC (I opened a thread because an issue with it) I experienced an issue with the MoBo, which wouldn't recognize my GPU in the first PCIE Slot. At the store they told me that it was a Bios config. and got it working.
Recently, I installed a Be Quiet cooler. Since it is too big, I removed the GPU, installed the cooler then reconnected all. To my surprise, the MoBo wouldn't recognize the GPU.
Here is the weird stuff:

- If I turn off the PC, connect a cable to the OnBoard HDMI port, the PC gives me image (Even boots to Win10), but it states that no GPU is installed.
- If I go to the Bios and change the primary output to any other value but CPU, disconnect the HDMI cable and save the changes, it detects the GPU and outputs video.
- If I reboot the PC (Or turn it off), it stops detecting the GPU, again.
(It's hard to explain, so I'll try to publish a video later, if I get the chance)

Something I noticed this time is that, when it detects the GPU, it states that is running at 8x speed, instead of 16x.
I tried updating the Bios, but no luck.

I do the steps I mentioned before as a Workaround, but it gets quite annoying. My guess is that the MoBo is the one with the issue since, the first time it happened, was before buying my actual GPU.

MoBo: Asus Prime-H370 Plus
Proc: Intel core i5-8600
GPU: EVGA Geforce GTX 1080ti
Ram: 4Gb Crucial x2; 8Gb HyperX x2 (24Gb total)
PSU: EVGA 1000W GQ 80+ Gold

If anyone happens to know what can be causing the issue, or has experienced something similiar, I'd be grateful if you could share your experience and how got it fixed.

Regards :)

Update
Enabled both external and integrated graphics. The result is quite interesting, resulting on the MoBo detecting when it wants the external GPU. It's kinda the same behavior I described in my reply, but the video gives more insight of the problem I'm facing.
[video="https://www.youtube.com/watch?v=Q4ntRzqcyes"][/video]
 
Jul 24, 2018
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It seems It has something to do with the MoBo, definitely. I tried my GTX 970 in that MoBo and the PC does not have any issue regarding to displaying something, but still says 8X instead of 16x. Also tried my 1080 ti on my old PC (Cannot remember the MoBo brand/model, just that Is compatible with 4th gen Intel procs and the socket is also 1151) and the GPU not just works, but it runs at 16x. This puts my Asus Prime H370 plus in a really bad spot, since it seems that there's something wrong with the slot. But still It does not explains why it does detect the 1080ti when I Switch the main GPU in the BIOS and disconnect the cable from the On Board GPU.