iMac 27" 5K Late 2015 screen artefacts on shutdown

Hi Chael, same happens to me (same iMac model as yours but with 16GB, sierra 10.12.1). Occurred randomly twice last week. I thought it had something to do with heavy usage (I did some stuff with final cut pro, garage band, Photoshop and iTunes) but this morning it happened again after a simple cold boot, login, opened up Mail.app, printed one mail, did shutdown. Quite sinister to be honest.

I have the same issue occurring on a clean install of a brand new late 2005 iMac 27" I bought two weeks ago. This is using m395x with 4GB graphics memory and 16GB ram if that bit is of any help. The only way I was able to remedy the problem was to physically unhook the mac from electric outlet for a bit and starting it back up.

Was the clean install a Sierra install?

You may find that it goes and comes back randomly.

I'm tempted to do a clean install of El Capitan as I believe it's a driver issue. However I am starting to read about late 2015 iMacs 27 inch 5k being repaired because of this and it could be a power related issue, which doesn't sound great.

Yes, clean install was a Sierra install. I'm on 10.12.1 right now.

I've got this issue as well.

https://youtu.be/A8laYDwAP4w

I sometimes even get a domino effect:

https://youtu.be/L04qK-PYE3s

I've the artifacts same as first video. Nothing the second.

I try a OSX 10.11 to external disk, I worked on it for two weeks, no artifacts on it.

I'll see if 10.12.2 resolve the problem, otherwise I downgrade as soon as possible.

I also get this strange colored horizontal lines artifact as I restart!

I really hope that Apple are aware of this and pursue a fix. I would just encourage everyone to log this on the feedback so they are aware.

Exact same here - 27" i7 32GB RAM 4GB M395X.

Only started happening after Sierra upgrade. Been on to support and done the SMC and NVRAM resets - no change.

They have advised me that it sounds like a hardware issue and to take my machine into an Apple store. However, from reading this thread it would seem unlikely to be a hardware issue - everyone is having the same problem, and only after upgrading to Sierra. I would be interested to know if anyone who did a clean install of Sierra is also having this issue - or is it just those who did the upgrade?

Clean install does not help. The issue doesn't appear on El Capitan. I'm on my 3rd brand new iMac 27" 5K 2TB machine and it continues. Ensure you have the issue elevated to Apple Engineering as its the only way it will be fixed.

Thanks for that - saved me the trouble of a clean install! Why have they replaced yours twice? Was it for this issue? How do I get it elevated to Apple Engineering?

First machine purchased through authorised Apple retailer. After going through the whole saga with Apple support I returned thinking it was a hardware problem with that machine. Second machine purchased from Apple retail shop - same problem - returned after 7 days. Third machine purchased from authorised Apple retailer with intention to use El Capitan. However I could not transfer my data from Time Machine or my old 2009 iMac because they were on Sierra. I had no choice but to upgrade the new machine to Sierra in order to transfer my data.

Keep telling Apple support of the problem - direct them here - and insist that it has to be elevated to Engineering. The Apple Store is not interested in fixing the problem as they cannot.

I bought two brand new 27" 5k, and as you have described, both have this issue. screen artefacts upon shutdown.

Same issue with my 2016 iMac 27" 5K Retina

Link to report problems to Apple?