2014 27” iMac Retina Graphics Glitches only when external monitor is NOT plugged in?

[cakeoflongness]

Hi guys,

my 2014 27” iMac (retina 5k) has been experiencing display glitches such as freezing, screen going black, turning off and rebooting etc.

I recently discovered that none of these problems occur when I have an external monitor plugged in..
Could it be due to the graphics switching from onboard to dedicated?

anyways, I’m wondering if there’s a way that I can trick the iMac into thinking there’s an external monitor plugged in or permanently switch the graphics over to the non faulty ones?

I’ve tried a basic dummy plug however this doesn’t fix the issue, only another monitor.

Any help or advice is greatly appreciated! Thanks in advance!

 

[Juicy Box]

Are you doing screen mirroring?
What is the resolution of the external monitor?

I wonder if the iMac is using a lower resolution when the external monitor is plugged in. If this is the case, maybe the GPU is struggling to push 5K, but fine when the resolution is lowered.

 

[cakeoflongness]

Are you doing screen mirroring?
What is the resolution of the external monitor?

I wonder if the iMac is using a lower resolution when the external monitor is plugged in. If this is the case, maybe the GPU is struggling to push 5K, but fine when the resolution is lowered.

Thanks for your reply! Interesting point!
Not doing screen mirroring, the external monitor is a 1920x1060 60Hz running through a MiniDp to VGA adapter.

Is there a way to tell what resolution the iMac is running in? in Display settings its set to Default for Display.

Also, I can play 4k video through YouTube perfectly.

Hope that helps!

Thanks again

Edit: Have attached the resolution when running the external monitor, and is the same without the external monitor.