Chosen Solution
Late 2012 Imac A1419 EMC2546 after trying to wake it up gave me a cloud of dust as fans kicked in at a full speed and screen is black.Power button works turns iMac off and on but fans keep running at a full after it turns on.Screen is black with no signs of life…after turning computer off i could hear some slow fan running … …at the moment have a screen removed and trying to find out if some heat sensor giving me this problem ….
Try an NVRAM reset (Option+Command+P+R), typically resolves display issues if it isn’t a hardware issue with the display itself: https://support.apple.com/en-us/HT204063 Also try an SMC reset: https://support.apple.com/en-us/HT201295 If that gets the display running, next run Apple Diagnostics, which should show any issues with thermal sensors: https://support.apple.com/en-us/HT202731
