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Model A1419 / EMC 2806 / Late 2014 or Mid 2015. 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (iMac17,1) All with Retina 5K displays

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Screen flickering and then shut down

Got this problem recently on my iMac 27” late 2015 model.

The iMacs screen start to flicker briefly and then the computer shuts down. Some times it can run OK for about 10 to 15 minutes, but some times it can only do for a couple of minutes.

It tries to reboot and then shuts down again but sometimes it just stay dead. I have to pull the plug from the socket and reconnect it again to get it to boot.

I have run the Apple diagnostic tool by pressing “D” during it boots up, and it returns:

No issues found

So, what could be wrong before I open the screen assembly for further check? Any thoughts or comments?

Many thanks.

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3 Comments:

Sadly, diagnostics won't test everything. Power supply and displays being the ones you could be facing here.

Can you give us a bit more here. Did you even have the system serviced or opened to add or replace something?

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Bought it over market place, and the previous owner did open it and have SSD upgraded. By the way, I have the screen assembly opened again, and have it connected with my Thunderbolt Display. Here is what I found so far:

1. Pictures can be displayed on external monitor correctly (thunderbolt display).

2. But the fan is too loud, I have to use the fan control software to bring it down a bit. Not sure if this is due to the display assembly has been removed and lead the iMac in "open" environment.

3. CPU has 70% occupied and most of it are from "kernel_task", not sure what it is, but looks like I can not stop it manually.

@danj

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One more thing I also found is that one of the metal plate on the right side of edge (there are two there with wires under it) are loosing and found one end was broken, not sure if this will cause the problem or not.

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OK, much better!

So the fan ramping up and the kernal_task running hard is all do to the lack of sensors feeding SMC with data. SMC goes into CPU Safe Mode ramping the fan up and lowering the CPU’s clocking to a crawl to prevent it from overheating. Once you get the sensors working and reconnected properly your system will be much better!

So we know the person who opened the system up before you messed up the display so you’ll need a new one 27” iMac Retina 5K Display Assembly (Late ‘15), Apple P/N 661-03255

It also sounds like you have one or more antennas messed up as well here’s a listing for them:

Block Image

Here’s the guide to put the display in iMac Intel 27" Retina 5K Display Display Replacement

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18 Comments:

Hi Dan,

I just tried to swap the display with my friend's iMac(same model), and the display assembly from mine works on my friend's iMac perfectly without noticing any display issues at all. So, it looks like the display assembly should be OK, right?

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I haven't tried his display on mine yet, but will try it and see what I can find more. Will keep you updated.

by

Tried the other display assembly on mine and the flickering issue came back, so, what else could it be?

by

Hi Dan, have tried several more tests in the past 2 days, here is what I found:

1. notice the previous owner installed mixed brand ram, and it is the slower one pc3-12800 instead of pc3-14900, so, removed some of them and just leave the same brand ram there, it can run normal a bit more time, but eventually same problem (screen flickering, and shutdown or restarting)

2. Changed the apple pc3-14900 ram and run it, same issue but got the following error message:

MCA Error Report, CATERR detected! No MCA data found.

Any ideas or what else I can look at?

And by the way, is there a way to setup iMac with Thunderbolt Display monitor and let the image only showing on thunderbolt or only use the external monitor?

Thanks,

by

I would try swap out of the power supply https://www.ifixit.com/Store/Mac/iMac-27... otherwise your logic board is suspect.

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