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petra wanted to know if her old gaming PC was working, since it hadn't been used in a while.

i unplugged my PC, hauled the 15kg hunk of metal out of the way, set her 10kg hunk of metal up, plugged in the seven different cables it needed, turned it on... everything worked perfectly! even the wifi card that i'd lost the screw for and stuck down with sticky tape!

except... only one of the monitors worked. the HDMI one worked, the DVI one didn't. after i updated her OS from fedora 36 to 38 to 40, i tried a whole bunch of things. the monitor was working fine before i swapped the PCs, so i tried moving the cable between GPU ports - no dice. i powered it off, unplugged the HDMI, and booted into UEFI with just the DVI connected - nothing. the monitor states "the cable is connected, but your PC isn't sending a signal".

i spent an hour and a half researching xrandr, the kernel DRM structure, DVI-D vs DVI-I, dual-link vs single-link, different graphics modes, driver and firmware updates, spurious kernel message logs... nothing could fix it. i declared the GPU's DVI ports dead. i knew she was having some weird graphics issues when this computer was last used (over two years ago!), so i chalked it up to that.

then, i swapped the machines over, set mine back up, and... no output on the DVI monitor! is my trusty decommissioned dell monitor that i got for free from university ewaste finally dead?

i reached behind the monitor, wiggled the DVI cable, and it sprang to life.

so, uh, just because the monitor says "the cable is connected", doesn't necessarily mean it's properly connected.

the moral of the day is: make sure you try the dumb things first. question the most basic assumptions you make when debugging - if i hadn't assumed "it was working before, so it must be plugged in properly at the monitor end", i would've saved myself a ninety minute goose chase.