Chosen Solution
I am repairing my A1418 iMac late 2015 EMC 2833, and I can’t seem to find a proper solution to the problem. On boot, one of two things happen. Either it goes straight to looping the startup chime and the fan speed goes crazy. There is also no display output. The other thing that happens is sometimes, I can get to internet recovery mode as the hard drive has been wiped, but after between 5 seconds to 10 minutes, the computer freezes and goes back to looping the startup chime with no display output. I have tried SMC reset, PRAM Reset and NVRAM Reset. Attached is a video of what happens when it boot loops. The video also shows the diagnostic lights. In fact, the fan was so loud, it is hard to hear the startup chime. EDIT -
I ended up using a Linux USB to go in and erase everything. Now, doing an internet recovery it has been 30 minutes, and it is restoring itself. It has yet to freeze up, knock on wood.
The fan will go into high RPM as your display is not connected this is to protect your system from over heating. So that is expected. As for your constant chime gets into the failure of post. Review this great write up on the process Visualizing the Mac Startup Process: What Happens When a Mac Boots? and Booting the Mac:Visual Summary You first need to reconnect the display for now (use some painters tape to hold it on) Now you’ll also need a bootable external drive so we can remove the internal drives OS and drivers. To get to it you’ll need to restart the system and press the Option (⌥) key to gain access to the startup disk menu to then select your external. Hopefully you can get to that point. Reference: Mac startup key combinations If you are still not able to do that lets try the Target Disk Mode command which is enabled sooner in the process. This time restart your system and press the T key are you able to get your system into Target Disk Mode. You should see the Thunderbolt symbol on the display. You should use a USB Apple keyboard as well. Update (03/16/2020) Good! Having a second system will make this easier. Leaving your bad system in Target Disk Mode we only need to connect the two systems back to back via a Thunderbolt cable. Basically the iMac system in Target Disk Mode will look like an external drive to your MacBook Pro. Using your systems Disk Utility you can wipe the drive reformatting the drive and then run the OS installer. If you can’t connect your two macs together (don’t have the needed cable) you can also setup a bootable OS installer thumb drive. References: Use target disk mode to move files to another computerUnderstanding the Applications for Target Disk ModeFYI: If you’ve got an old macOS install image, it will probably stop working todayHow to upgrade to macOS High Sierra Use this installer!Format a Mac’s Drive Using Disk Utility (OS X El Capitan or Later)How to Clean Install MacOS High SierraHow to create a bootable macOS High Sierra installer drive
