Originally Posted by
broomdodger
If you have a wireless keyboard...
find a WIRED keyboard for the following.
Unplug ALL peripherals
Plug in ONLY a wired keyboard
Reset SMC for iMac
Shut the iMac down
Unplug the iMac from power
Wait 15 seconds with the iMac unplugged
Plug the power cord in and wait 5 seconds
Press the power button
Immediately hold down the COMMAND and letter S keys
Until you see a black screen with white letters
Did you get to a black screen with white text?
Do you still see vertical lines?
If yes, then it is a bad GPU.
Force quit the iMac:
Hold the power button until iMac shut down.