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Why would a video card stop a PC from starting

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Long story short: New build, not the latest parts. Turns out swapping a known working card allows the PC to boot. Way past RMA time.

Could this be something simple or is it most likely not worth thinking about?

Try different slot, try without other PCIe devices, try after resetting BIOS settings.

PC BIOS 'needs' to see a video card, if it doesn't then it won't complete POST.

As Magic says, take the PC down to bare minimum, try a different slot.

If the PC won't POST with the card in it could be a faulty card or a corrupted video BIOS on the card, you may be able to flash the BIOS on it if the PC boots with the 'faulty' card and a known working second card

I guess I left out the part where it is completely dead with the card in it. I originally thought it might be a bad psu, but that tested good and then I ran out of time to mess with it. I needed to get a laptop worked on the other day and brought this along for them to check. They told me it ran just fine with a different card, The only other one I have is ancient and might not work anymore anyway. I thought I would just get another card, but they seem to be in short supply right now. I was wondering if maybe there was something I could look for on the card that was keeping it from working. I know it is a long shot, but figured I would ask.

Nominal Animal:
A short in the card connector in a suitable position can do that.  I do believe I once had a tiny piece of swarf (from the stamped chassis) in a VESA VBE connector cause something similar: the machine would not even turn on.  On PCI Express bus, I guess this could only happen between ground and one of the power pins, but the pinout shows it does have power and ground pins next to each other in more than one spot.  And metal swarf in PC chassises is very, very common: the stamping process often leaves hanging whiskers, that only later detach and drop onto the motherboard..

Internally, PCI Express bus consists of lanes, with serial unidirectional transfers.  The card can garble the operation of the system by sending continuous interrupts (say millions of times per second) or not answering some of the fundamental messages sent, causing the system to "lock up", somewhat as if keeping the system stuck between clock cycles.

It has always been the case on x86 and x86-64 that an expansion card can render the entire system nonfunctional, even stop the CPU from starting (BIOS execution).  Graphics cards especially so, because most BIOSes expect to see one, and won't boot without; they're really not prepared to deal with or even detect a misbehaving graphics card (wrt. bus communications or interrupts etc.).  Because BIOS is on a Flash chip on the motherboard, and all this occurs immediately on power on, way before any OS or boot loader is involved, none of this has anything to do with the OS used at all.


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