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smc_lid was the first thing I checked, it's present. strangely, if I close the lid, the LED will start breathing like it's in sleep mode, and when I open the lid, the LED turns off, but no video of course
yes, prior person replaced LVDS and ran a jumper for bklt for some reason. thankfully the guy didn't clean anything off, so the area around the gpu was covered in dust and no liquid made it under the chip or even on that part of the board, only LVDS side. only darkened probe points are a few between the lvds connector and the fan connector
with a usb drive, it looks like it's booting off it since the LED will blink for a while, but even plugged in for five minutes, it doesn't give me video. in target disk mode, the drives won't pick up on another mac either.
 hmm, after ten or so minutes, it shits itself off as well. neat
external with and without internal plugged in, can't get video at all. left it plugged in over night for shits and giggles and it was still on when I got to work this morning, so it's not shutting down at least. are the GPUs flakey in these like the 2011s?
These GPUs are not a known issue on these. You could enable remote login on the OS with a good machine and then boot this machine from that drive. You can then remotely login and see if the machine is booted and if the GPU is recognized.