Question PC won't post when GPU is connected to Motherboard, but it will when it's not plugged in.

Jan 29, 2021
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I just bought a new PC and I finished building it yesterday. When I first powered it on everything seemed to be working, until I noticed that the monitor did not receive any signal (the cable was plugged into GPU). I've now disassembled the PC and put it together outside of the case on top of the motherboard box. Without the RTX 3080 in the PC, it boots into windows but when I insert my GPU again it doesn't even POST. I tried popping in my GTX 1080, which I know is working since I'm using it inside the computer that I'm writing this post with, and that didn't work either... Is my motherboard or GPU broken? PSU is fine since I tried two different ones and the issue was still there.

I've reseated the GPU and RAM multiple times. I've downloaded the latest BIOS with a flash drive. I've reset CMOS. I can't get the computer to post with the GPU installed.

These are my specs:
CPU: I9-10900k
GPU: RTX 3080 MSI SUPRIM X
Motherboard: Z490 MAG TOMAHAWK
RAM: Corsair 2x8 GB 3200Mhz
PSU: Corsair RM850 850W (2019)
M.2 NVME 1: Samsung 980 Pro 500 GB
M.2 NVME2: Samsung 970 Pro 500 GB
CPU Cooler: Noctua NH-D15 chromax black

Thanks for any help!
 

thomas123321

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Since you did try another gpu on the motherboard and still didnt boot, it does look like its the motherboard. And you tried updating bios, resetting cmos. I believe the mobo has two pcie slots. Try the other pcie slot and see if it posts.
 
Jan 29, 2021
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Since you did try another gpu on the motherboard and still didnt boot, it does look like its the motherboard. And you tried updating bios, resetting cmos. I believe the mobo has two pcie slots. Try the other pcie slot and see if it posts.
I tried both PCIE slots but unfortunately that didn't fix it.

I also tried putting the card in my old computer since I know that every component is working, but the case was too damn small. I took out the motherboard, CPU, SSD, PSU and GPU from the old case and tried to turn it on but the system was stuck in a boot loop (maybe because it recognized that some parts were missing?). If the old system would have booted, I would have been able to find out if the 3080 worked but for some odd reason, it didn't want to. I'll take the new system to a computer shop tomorrow and let them test the mobo and 3080.
 

thomas123321

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I tried both PCIE slots but unfortunately that didn't fix it.

I also tried putting the card in my old computer since I know that every component is working, but the case was too damn small. I took out the motherboard, CPU, SSD, PSU and GPU from the old case and tried to turn it on but the system was stuck in a boot loop (maybe because it recognized that some parts were missing?). If the old system would have booted, I would have been able to find out if the 3080 worked but for some odd reason, it didn't want to. I'll take the new system to a computer shop tomorrow and let them test the mobo and 3080.
Yea that's a good idea, btw when you tried the 3080 on you older system, did you remove the drivers completely? If you didn't try removing them using DDU. Search YT if you want a tutorial, there's many.
 
Jan 29, 2021
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Yea that's a good idea, btw when you tried the 3080 on you older system, did you remove the drivers completely? If you didn't try removing them using DDU. Search YT if you want a tutorial, there's many.
I did not since I couldn't boot. Maybe there's a way to do it without booting, but I don't know how to do that. Thanks for the idea, but I rather not disassemble my PC for a third time today :p
 

thomas123321

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I did not since I couldn't boot. Maybe there's a way to do it without booting, but I don't know how to do that. Thanks for the idea, but I rather not disassemble my PC for a third time today :p
Yeaa, the 3080 probably didnt boot on you older system cuz of the gtx1080 drivers that was already present.

I'm suspecting the mobo to be the faulty. Anyways keep us updated. Cheers
 
Jan 29, 2021
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Yeaa, the 3080 probably didnt boot on you older system cuz of the gtx1080 drivers that was already present.

I'm suspecting the mobo to be the faulty. Anyways keep us updated. Cheers
Fixed it! I took the computer to a local computer shop and the guy working there took a quick look at the system and he asked if the white cables that I used to power the GPU were included with the PSU in the system. I answered that they were from another PSU and he said that the problem was most likely that. I bought 3 new PCIe 6+2 cables that were compatible with my PSU model and plugged them in and VOILA! Everything is working now and FINALLY, I can enjoy my RTX 3080 and I9-10900K. Thanks for the help!
 

thomas123321

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Apr 28, 2013
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Fixed it! I took the computer to a local computer shop and the guy working there took a quick look at the system and he asked if the white cables that I used to power the GPU were included with the PSU in the system. I answered that they were from another PSU and he said that the problem was most likely that. I bought 3 new PCIe 6+2 cables that were compatible with my PSU model and plugged them in and VOILA! Everything is working now and FINALLY, I can enjoy my RTX 3080 and I9-10900K. Thanks for the help!
Ayy, didn't expect the cables to be the problem here. Great to hear that was the problem and not the mobo. Well good luck with the build, tc
 

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