27" late 2013 iMac resolution has changed, and will not change back

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27" late 2013 iMac resolution has changed, and will not change back

Hi,

I've recently had my iMac unplugged and off for a month or so. Previous to this the display was perfect at 1920x1080p and everything was crystal clear. However since turning it back on, it has changed my display resolution to 1080x720p and the only other option (after alt + clicking 'scaled') is 800x600.

All of my icons are huge, everything is pixelated and cannot seem to find a way to 'add' back in the 1920 x 1080 resolution to the 'scaled' list.

I've restarted and checked for software updates, and its updated to the latest it can handle (macOS catalina 10.15.7 (19H2026)). I've also connected to a new display via HDMi and the second display seems to work perfectly. I've also checked the accessibility options and toggled with the zoom feature, but it only allows me to zoom in more, not out.

One thing i've also noticed. Is that when i now turn it on, the initial black screen shows some lines of bright pixels that seem to flicker, almost as if its a damaged screen, but then loads up the OS and works without any 'damage' symptoms. (iMac was also stored safely and upright whilst it was unplugged)

This is really racking my brain and can't seem to find a fix on youtube or online anywhere else.

Thanks in advance.

Earlier Mac models

Posted on Jun 26, 2024 6:10 AM

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Answer 1

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den.thed

Level 10

92,992 points

Jun 27, 2024 6:33 AM in response to Rossharrison11

You still have Mirroring checked in the Arrangement tab.

As I explained before.

When Mirror Displays is on, the macOS tries the best match the resolution of both Displays.

If you want the iMac display and the external display to run at their native resolutions.

Then you need to uncheck Mirror Displays and use Extended Desktop Mode.

Please do the above and test, before you do any resetting, erasing or restoring.

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Answer 2

den.thed

Level 10

92,992 points

Jun 26, 2024 9:15 AM in response to Rossharrison11

Hmm.. that is strange.

Has this iMac ever had the Display, GPU or Logic Board replaced...?

Does another Display show up when you click on the Arrangement Tab...?

Also strange.

The native resolution for a late 2013 27" iMac is 2560-by-1440

and the native resolution for a late 2013 21.5" iMac is 1920-by-1080

With that in mind please verify the exact size, year model and spec's of your iMac.

see > Identify your iMac model - Apple Support

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Answer 3

den.thed

Level 10

92,992 points

Jun 26, 2024 7:19 AM in response to Rossharrison11

Disconnect any Mini DisplayPort adapters and cables from the iMac and reset the NVRAM.

see > Reset NVRAM on your Mac - Apple Support

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Answer 4

hcsitas

Level 6

13,970 points

Jun 26, 2024 9:44 AM in response to Rossharrison11

To me it looks like your Mac is getting ready to say goodbye forever. IIWY, I’d immediately copy all user data files (PDFs, jpegs, docx-es etc.) to an external and work off the external from this point on. The “external” can also be iCloud. Do not use TM for this - it could hasten the demise or do a half-job.

Once you’ve taken the above step, try and locate an Apple-authorized repair center willing to swap out the power supply - that’s what IMO is dying the slow death and causing your issue.

Happy to be proven wrong, all the best anyways.

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Answer 5

den.thed

Level 10

92,992 points

Jun 26, 2024 9:48 AM in response to Rossharrison11

When Mirror Displays is on, the iMac tries the best it can to match the resolution of both Displays.

If you uncheck Mirror Displays and use Extended Desktop Mode, then you should be able to select the native resolution for each Display.

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Answer 6

hcsitas

Level 6

13,970 points

Jul 18, 2024 1:19 AM in response to Rossharrison11

Reset to factory and set up as new. Be sure to copy user files to external without involving Time Machine before factory reset. You would also need to make a note of important System Settings for replication later. Once your Mac comes up as new, recreate accounts, re-install apps and copy user files back. Good luck!

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Answer 7

den.thed

Level 10

92,992 points

Jun 26, 2024 7:53 AM in response to Rossharrison11

What happens when you select "Default for display"

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Answer 8

hcsitas

Level 6

13,970 points

Jun 27, 2024 1:51 AM in response to Rossharrison11

A bigger reboot in my book is reset to factory and set up as new, which would mean recreate accounts, re-install apps and copy user files from external without involving Time Machine. You would also need to make a note of important System Settings for replication later. Good luck!

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Answer 9

Sep 10, 2024 1:38 AM in response to Rossharrison11

*UPDATE*

This has now been resolved. I went away for the weekend and left my imac on (in sleep) by accident. Come home and woke it up to a lot of mac pop up windows asking for my password (namely icloud) and a few things seeking permission, I hit cancel for them all and there must have been 30+. After that they were popping up intermittently, and the mac ran very slow between them, I had to hold the power button to kill the machine, booted it back up, the pixelated apple logo showed as it had been with the issue, and at 75% of the bar loading, it popped back to its correct resolution. The desktop loaded up, Pixelated as before, but the difference being, the correct resolutions were available in the display settings once again, i hit 'default for system' and it's back to being perfect again!

So to summarise, i actively did nothing to fix this, it did resolve itself, after 3 months of having the issue. Sorry there is no clear answer for anyone with this issue, but maybe it's not time to give up hope after all!

Thanks for those who offered advice

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Answer 10

den.thed

Level 10

92,992 points

Jun 27, 2024 11:39 AM in response to Rossharrison11

Good info, thanks for updating.

Now we need more detailed information about the external display and how it is connected the the iMac.

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Answer 11

den.thed

Level 10

92,992 points

Jul 18, 2024 6:39 AM in response to Rossharrison11

Rossharrison11 wrote:

Hey, don't suppose you've had any more thoughts on this at all have you please?

What happens when you unplug the display adapter from the iMac and restart...?

I'm asking because it is starting to sound like there is a problem with the Mini DisplayPort to HDMI adapter.

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Answer 12

Jun 26, 2024 9:49 AM in response to den.thed

I get the other display show up on 'arrangement' once i've connected it yes, but once disconnected then no it just focuses on the iMac..

Never had anything replaced, but only a SSD fitted from apple when i bought it back in 2013.

this is my specs...(currently have the second display plugged in)

[Edited by Moderator]

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Answer 13

Jun 27, 2024 12:55 AM in response to den.thed

Done that, and can select different resolutions, but still don't get a larger resolution than the 1280 x 720 its stuck on for the imac. if i DO mirror them, i can select the 1080p for the second monitor, and that does zoom out the imac screen, but everything still seems pixelated and doesn't sit quite right still. (picture attached for reference)

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Answer 14

Jun 27, 2024 11:31 AM in response to den.thed

yes i only checked it on to take that screenshot showing that the screen did go back to some form of 1920 x 1080p display but was still pixelated, i have mirroring checked off all the time otherwise. the screen below shows the two monitors with their separate scaled available resolutions

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Answer 15

Jun 29, 2024 11:52 AM in response to den.thed

It's connected with a thunderbolt to HDMi adapter, and then a HDMi to HDMi cable. However, this mac had never had an external monitor attatched until this issuse arose, I only connected the external monitor to test to see if it was the screen, or the macOS system. so this external monitor had only been connected after this issue started. And i'd never been into or changed the display settings since purchase.

thanks

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27" late 2013 iMac resolution has changed, and will not change back

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