Chosen Solution
the system was in a sleep mode for a 6-7 hours until i needed to use my iMac…pressed a keyboard Space button and white puff of dust showed up …almost freaked out “Computer on fire!” was sitting and waiting and nothing happened black screen and no reaction neither mouse or keyboard.Tried to connect Apple wired keyboard from my 2009 iMac to see if there is any difference…none. Decided to open my iMac and blindly check the LCD skin sensor which did not measure with my multimeter.Ordered new one and new one does not measure either….might be my multimeter costing me $10 ,that’s what cost was for a new sensor… Tried the trick to unplug the power cord and replug from behind…… ordered thunderbolt/hdmi adapter with no results to see. Power button at the back of iMac is very responsive. Is there a possibility iMac is not sensing external display?
With the display removed locate the onboard diagnostic LED’s which is just below the power supply data connection cable:
What are they telling you when you power the system? LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty. LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly. LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement. LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.
