I recently upgraded my system from a Ryzen 5 2600X to a Ryzen 7 5800X3D to take advantage of what may be the last chance to max out this AM4 PC, and while I eventually got everything working, I ran into a very unusual issue that I haven't been able to find anyone else describing. I'm hoping someone here has seen this before or can explain what's happening.

System

• ASRock Fatal1ty X370 Professional Gaming (Rev. 5.1)
• BIOS 10.50 (upgrade path was P5.30 → P7.00 → P7.30 → P10.50)
• Ryzen 7 5800X3D
• Corsair H100i v2
• Gigabyte RTX 3060 Ti Eagle OC Rev. 2.0 (GV-N306TEAGLE OC-8GD)
• 32GB (4x8GB) G.Skill Trident Z DDR4-3200
• Samsung 960 Pro 512GB (M2_1)
• Samsung 970 EVO Plus 2TB (M2_2)
• EVGA SuperNOVA 650 G1
• Windows 10 Pro 64-bit

Symptoms

After installing the 5800X3D, the board would consistently stop with Dr. Debug code d6.

After some research I discovered the hidden fTPM prompt that appears after changing CPUs. I blindly pressed Y and Enter, after which the machine would continue booting into Windows, but there was still no video output from the GPU. The d6 error would still appear, it just no longer stopped there.

The PC appeared to be running normally, but there was no display on any monitor or GPU output.

Things I tried

• Updated BIOS all the way to 10.50
• Cleared CMOS
• Loaded BIOS defaults
• Enabled/disabled fTPM
• Enabled/disabled CSM
• Enabled/disabled ReBAR
• Enabled/disabled Above 4G Decoding
• Forced PCIe Gen3
• Tested with one RAM stick in B2
• Reseated CPU
• Inspected CPU pins and socket (no bent pins or debris found)
• Removed the second M.2 SSD
• Tried multiple HDMI ports and multiple monitors
• Swapped GPUs

None of those changed the behavior. However, when I would reinstall the 2600X CPU, it would boot normally.

The breakthrough

Out of desperation I installed an old Gigabyte GTX 1060 in the second full-length PCIe slot.

The system immediately booted normally.

Windows only detected the RTX 3060 Ti.

Even more interesting, I found that the GTX 1060 didn't even need its PCIe power connector attached. Simply having the card physically installed allowed the system to POST normally.

That led me to experiment with the BIOS.

Current workaround

Changing the PCIe Lane Configuration from x16 to x8/x8 completely resolved the issue.

Now I can remove the GTX 1060 entirely, leave only the RTX 3060 Ti installed, and the machine boots normally every time with no d6 error.

GPU-Z confirms the RTX 3060 Ti is now running at PCIe 3.0 x8. (see image)

Current status

The machine is stable.

• Windows 10 Pro
• Cinebench stable
• Star Citizen (the game I play most) is stable with all graphics settings on High
• FPS lows improved from roughly 20 FPS to around 30 FPS
• Capped at 60 FPS on my display running at 1080p

So while running at PCIe x8 isn't ideal, it's still a huge improvement over my old 2600X.

My current theory

My current theory, since I don't have a spare CPU/MB, is that this is some kind of PCIe x16 initialization/training issue specific to the Fatal1ty X370 Professional Gaming when paired with a Ryzen 7 5800X3D. Forcing x8/x8 appears to make the firmware take a different PCIe initialization path that avoids the d6/no-video problem. I am open, even hoping, to being wrong.

Has anyone else seen this behavior on this board or another X370 board?

More importantly, has anyone found a way to restore stable x16 operation without forcing x8/x8?

Any constructive feedback or ideas would be appreciated.

https://i.redd.it/p3qy20gpwlbh1.gif