No power. iMac won’t turn on.
My iMac won’t turn on. No power..
It sounds like you have a blown power supply. If resetting the SMC doesn't fix things you'll need to either bring your system into an Apple Store or authorized service center to get the power supply replaced.
Opening the newer 'Thin Series' is a bear without the correct tools and not following the steps to the letter! You can quite easily damage the display just in taking it off. So think long and hard before you attempt opening your system.
OK! I still was to try to repair my system my self. Review this IFIXIT guide: iMac Intel 21.5インチ Retina 4Kディスプレイの交換 Make sure you get the tools and a few spare adhesive strips.
Once you open the system you'll need to check the onboard diagnostic LED's
LED 1 - Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will turn on when you connect the iMac to a working AC power source. The LED will remain on as long as the computer is on or asleep.
When the computer has been shut down correctly, LED 1 behavior may differ:
If a startup event is scheduled in System Preferences/Energy Saver, LED 1 will stay on after a correct shutdown. If no startup event is scheduled in System Preferences/Energy Saver, LED 1 will turn off and will stay off as long as the power cord is kept connected and an AC power source is present. Disconnecting the power cord and plugging it back in will turn this LED back on, even if the computer is still off.
After disconnecting and reconnecting the AC power source, this LED could remain off: If the AC power source is missing or disconnected.
If the logic board is disconnected from the power supply or the AC receptacle.
If the power supply board is faulty.
LED 2 - Indicates that the computer is turned on. This LED will be on as long as the computer is turned on (but is not asleep) and the power supply and voltage regulators are working correctly.
LED 3 - Indicates that the computer and 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, the backup battery (on back of logic board) may need to be reseated or the video card might be installed incorrectly or need replacement.
LED 4 - Indicates that the computer and LCD panel are communicating. This LED will be ON when the computer is turned on and a video signal is being generated. If LED 4 is on and there is no image on the display, the LCD panel or the cables between the LCD and logic board might be installed incorrectly or need replacement.
LED 1 = Power is available.
LED 1 + LED 2 = Power is available and the system is turned on. If the second LED is not visible when the power button is pressed:
LED 1 + LED 2 + LED 3 = Power is available, the system is turned on, and the GPU was found.
LED 1 + LED 2 + LED 3 + LED 4 = Power is available, the system is turned on, the logic board is communicating with the GPU, and the internal LCD was found.
Try this it might help.
Shut down Mac and plug in the power cord.
Press Shift-Control-Option, then press Power button.
Hold the keys for about 10 seconds.
Release the keys.
Press Power as you would normally start your Mac.
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