Jonathans Blog

LG 34um95 Yosemite resolution bug

Jonathan Mitchell
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The LG 34UM95 was one of the first truly compelling ultrawide monitors — a 34-inch 3440×1440 IPS panel with Thunderbolt connectivity that made it particularly attractive to Mac users. The catch was a resolution bug that appeared specifically with OS X Yosemite, where the monitor would not correctly report its native resolution and macOS would offer a reduced set of scaling options. It was a frustrating issue for an otherwise excellent display.

The Bug and the Fix

The issue stemmed from how Yosemite handled HiDPI display detection via Thunderbolt — the monitor’s EDID data wasn’t being read correctly, causing macOS to fall back to scaled resolutions that didn’t reflect the panel’s actual capabilities. The fix involved either using a DisplayPort connection rather than Thunderbolt (which bypassed the detection issue), or using a third-party utility like SwitchResX to manually add the correct resolution to the system’s available modes. Neither was ideal, but both worked reliably.

Verdict

Ultrawide monitors and macOS have had a complicated relationship at times, and the LG 34UM95 Yosemite resolution bug was an early example of the growing pains involved in pushing display technology forward faster than operating system support can follow. The workarounds were effective, and subsequent OS X updates eventually resolved the underlying issue. A useful troubleshooting note for anyone still running this specific hardware combination.

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18 responses to “LG 34um95 Yosemite resolution bug”

  1. Jay Avatar
    Jay

    Hi Jonathan, thanks a lot for this blog post. I’m on a mid-2012 MBA and had been happily running a 34UM95 at 3440×1440@50hz prior to upgrading. I tested the Yosemite beta previously and had the issue of no 3440 support, so I restored. I tried again with the full release hoping this issue was gone, but as you know it hasn’t. I had a really painful experience trying to restore my mavericks install the second time, I somehow ended up with a drive-less partition that could only be resolved by booting to a bootable USB drive and killing the whole partition. Before I go through the steps of installing Yosemite again, can I just quickly ask how you’ve found the fix you’ve detailed? Has it been stable?

    1. jonathan Avatar
      jonathan

      So far its been very stable, nothing complains, and everything works like it used to :D

    2. Barry Ghotra Avatar
      Barry Ghotra

      Could you please share how did you or what steps you had taken to make it work on the MBA 2012. I’ve the 34UM95 and can’t make it work at all with either the thunderbolt or HDMI cables (TB to HDMI adapter). the screen shows garbled when first connect and then the display keeps goning-in-out on my MAC. the same monitor works fine with the dell laptop. Any clues or steps to take. I was able to installt he split screen the true color apps on mac but no luck installing the ASM software. I read somewhere that it’s needed to run the MBA using TB cable. I have 10.9.2 with HD4000 GRAPHICS card. Any help would be great. will check for reply, if someone can be helpful.

      Thx.

  2. Stefan Wrobel Avatar
    Stefan Wrobel

    Thank you! I can now use my Dell P2815Q 4K display in HiDPI mode again!

  3. Frank Brennecke Avatar
    Frank Brennecke

    Hi,
    you should also mention that disabling kext signing could cause problems in case you perform a NVRAM/PRAM Reset. The kext signing flag is stored in this RAM, and resetting it will lead to a grey question mark instead of a working Mac. So one should restore the original Yosemite kext before performing the PRAM Reset. For more information see http://www.cindori.org/trim-enabler-and-yosemite/
    Regards
    Frank

    1. jonathan Avatar
      jonathan

      good to know :D

  4. Gustavo Avatar
    Gustavo

    Hi,
    I have an ASUS pb287q monitor and I did all the steps twice but no 4k resolution with my 2012 MacMini (entry model) even after going to system preferences, displays and clicking on “scaled” with the alt option key hold down. I’ve done a clean install of Yosemite and it still didn’t work. I’m wondering if I’m doing something wrong (I’m not a newbie and the steps are fairly easy to follow) or if my monitor is not compatible with Yosemite/MacMini. Unfortunately, I can’t try on any other 4k monitor. I’d appreciate any help.

    Thanks

    1. jonathan Avatar
      jonathan

      Not sure with other screens, but this fix may be screen dependant

    2. James Avatar
      James

      The HD4000 on board GPU is not capable of typical 4k resolutions. 3440×1440 is the max resolution possible with the HD4000.

  5. JShasta Avatar
    JShasta

    I have a early 2013 MBP w/ Intel HD Graphics 4000 and this fix is not working for me. Am I doing something wrong/is this not possible on my model?

  6. Neon43g Avatar
    Neon43g

    Hi there! Thanks for this instructions, but i still have some problems with my 34 inch screen. I run yosemite on MBP13 mid2012 with HD4000 graphics. I tried your instructions with both original 10.9 and patched kext file. After applying kext and restarting my mbp runs great with 3440px resolution. But after second restart or sleepmode i have that strange artifacts in bottom part of screen (looks like frame buffer issue). Do you have any suggestions on this? maybe i did something wrong?

    1. jonathan Avatar
      jonathan

      I’ve also noticed this behaviour – only after the system goes to sleep – I’ve resorted to setting my mac to not sleep or turn on the screensaver and using the button on the screen to turn the screen off – not the best solution, but its the best workaround I have just now

  7. Per Avatar
    Per

    Hi. I have ugraded to 10.10.2. But I still don’t get better than 1080p on my 13 inch MBAir late 2010 with NVIDIA GeForce 320M 256 MB.
    Cable is mini DisplayPort to DP on monitor Aoc q2770 pqu. So all specs are there but not the resolution. Bought the monitor to get 2560 x1440 but it does not work. Annoying! Rgs Per in Stockholm

  8. John Smith Avatar
    John Smith

    I have a macbook air mid-2012 (10.10.2), with the lg 34um95 connected by mini-displayport (mba) -> displayport (Lg34um95), I have this problem where its limited to 2560×1080 even with closed clam.

    I tried your fix without success, also with the ‘option’ + ‘click’ on ‘scaled’ but the 3440px option never shows up.

    Any ideas?

    Thx

    1. jonathan Avatar
      jonathan

      Hi John
      I only tried this with thunderbolt cables, but I can confirm that it is fixed when using thunderbolt (but the cables are expensive) :|

  9. cranckhead Avatar
    cranckhead

    Hi, I have a Mini Mac 2012 Server model (HD4000 Graphics) and run Yosemite 10.10.2 with 1 LG 34UM95 display (TB cable). I know the Mac Mini is capable of running 2 Apple TB screens (daisy chaining), would it be possible to hook up 2 LG 34 screens (daisy chaining)? Thanx!

    1. jonathan Avatar
      jonathan

      not sure, I only have one screen so cant test it with the daisychaining :|

  10. Daren Jacobs Avatar
    Daren Jacobs

    Where is this preached version of the Mavericks file that you speak of. Can you put it on your site?

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