Chosen Solution
Hello I have a problem with my Imac 21,5" A1418 EMC 2544 I have upgraded From El Capitain to macOS Sierra. After second restart it went i a boot loop with grey screen and boot chime over and over again. I have disconnected my LCD and connected a external screen same thing . When i boot up without internal or external screen connected no boot loop only 1 chime and then it runs high speed fan. My question is how do i reset my NVRAM without screens and EFI password set ( I know the password set it my self ) . Or can some one tel me what i can do about this or what part or parts i have to re-please so i have a working screen again . With Regards The Judge
Try to press and hold Command-Option-P-R on bootup until you see or hear something new. Also, do you have another Mac that is new enough to download macOS Sierra from the App Store? If so, you can use target disk mode to install macOS onto the iMac via another Mac.
Your system maybe just fine! Apple requires a full restart during the upgrade process a few times as the there is a firmware update as well as an OS update (I’ve had systems do four restarts to apply everything). The other problem I’ve seen is a HD which has problems go into a tailspin of restarts as the firmware update is not able to complete. The only way to overcome this is to boot up under an external drive or use a second Mac system to fix if possible your drive using Disk Utility by putting your system in Target Mode. Otherwise you may need a new drive.
