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Hello! Display went to black. But starts fine. Machine is reacting on change of speaker volume. Starts perfectly in terminal mode (but black). Alt-cmd-R.P reset doesn’t work out. External Monitor attached : nope. Running iMac as an external Monitor on a macbook: negative (maybe wrong cable, or the function for F1, F2 is disabled in sytem prefs.). What couldit be: Display or GPU? Usually, if a graphic card is saying good-bye, you see some errors on the screen before. But this time not. Just turned black some day when booting. I’ve removed the display , yet. Is there a diagnostic method of testing, weather display or graphic card is the problem? Or Backlight? Many thanks, W.
Try this see if you can access your iMac from the MacBook over an Ethernet connection. Click on the menu Go then Network do you see your iMac system? If you do try sharing the display option at that point you’er accessing the system virtually over the network. If you can that implies your GPU card is gone as you stated you can’t access an external display either. You’ll need a new GPU card. Follow this IFIXIT guide: iMac Intel 21.5" EMC 2428 GPU Card ReplacementIf you can’t see or access the your iMac over the network and you are sure you have a good connection with the cable then the Platform Controller Hub has failed which will require a new main logic board. Follow this IFIXIT guide: iMac Intel 21.5" EMC 2428 Logic Board Replacement
Try to plug another monitor to check the graphic card.
