I put together a mixed new-and-old PC build with an RTX 5090 Founders Edition, Ryzen 9 9950X3D, MSI B850M Mortar motherboard, 1200W Thermaltake Gold PSU, 2TB Samsung 980 Pro, and 64GB of DDR5 memory. The system boots into Windows using the CPU's integrated graphics, but the motherboard and Windows do not detect the 5090 at all. The card powers on, its fans spin, and its lighting works. I've already reseated the GPU and RAM and checked the power connections. The motherboard has no debug LEDs. The display works when connected to the motherboard, but connecting the HDMI cable directly to the 5090 produces no signal. Since this is a micro-ATX board, there is only one PCIe slot. What else should I try?
2 Answers
Double-check the physical installation and the power cable. Make sure the card is fully seated and the 12V-2x6 connector is inserted completely with no sharp bend right next to the plug. Also confirm the motherboard BIOS is up to date, since a newer BIOS may improve compatibility with the 9950X3D and RTX 5090. If the GPU still isn’t detected after a BIOS update and a different cable or PSU connection, testing it in another PC is the quickest way to rule out a defective card.
Use the integrated graphics to enter the BIOS, then check whether the PCIe slot is visible and manually set its link speed to Gen 5 instead of leaving it on Auto. You can also try disabling PCIe power-management options temporarily. Sometimes the automatic PCIe negotiation doesn’t work correctly with newer hardware.
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