My iMac is starting and than turning off
iMac keeps turning off
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Disconnect all external devices except the USB Keyboard and USB Mouse. Restart and Immediately hold the Shift key until the Apple Logo is presented. This is Safe Mode and it will do a Repair Disk, clear cache files and only load Apple Software, extensions and fonts. Loads slowly - normal.
Does the computer complete the loading of the operating system ? Is it functional ? Does the computer still switch off or fail to load completely ?
If it fails to load completely - How to reset the SMC of your Mac and perform this action three time consecutively.
Then perform Reset NVRAM or PRAM on your Mac and perform this action three time consecutively.
Then test booting normally. If it still fails to perform - Find an Apple Authorized Service Provider