LG 27UD88 'no signal' with Mac Studio M2 Ultra

Question

Level 1

4 points

LG 27UD88 'no signal' with Mac Studio M2 Ultra

I am currently running a 2023 Apple M2 Ultra with an LG Ultrafine display, I recently added a friends old 27UD88-W.AUS as a secondary monitor, but I am no plagued with a "no signal" on the secondary monitor no matter what I try.

I previously was having display issues where I could not get my screen to sleep. It went on for almost a year of unplugging everything EOD. This issue seems to mostly be caused by running Karabiner to run my keyboard, but if I turn of Karabiner and unplug the keyboard I am now able to get the screen to sleep without waking within the minute. (I had run "pmset -g assertions" and saw "bluetooth LE HID Activity" and "Karabiner DriverKit VirtualHIDKeyboard 1.8.0" were culprits so tried "sudo pmset -a sleep 1 disablesleep 0" after with no clear result)

I then had the Mac studio sleep over the weekend, but when I opened it up only the primary monitor worked.

I have taken the following steps, but have not found any action to get the LG 27UD88 to receive signal.

I have changed and checked cables using the lightning cable running my primary display, changed ports, etc...
I have restarted, updated IOS, updated LG firmware, and checked software
I have tried to detect monitor in system settings etc...

Is there a test or command I should otherwise be running? Thanks to anyone that sees this!

Mac Studio

Posted on Mar 29, 2026 10:32 AM

Reply

Answer 1

Mar 29, 2026 11:25 AM in response to l0r3nz

that LG 27UD88 display appears to be a 4K display at 10 bits/color(HDR) cable for refresh rates 56 to 61 Hz

interfaces include:

• 2x HDMI 2.0 -- provides a top step of 14.4 G bits/sec over ONE cable. that can drive your 4K HDR display at 50 Hz directly. (slightly higher with a compromise in colors) also could support TWO-cable connection.

• DisplayPort 1.2. -- provides top step of HBR2, 17.28 G bits/sec, supports 4K HDR at 60 hz without Display Stream Compression

• the only USB-C connector is a genuine USB-3.0 port. Taken literally, rated at 5 G bits/sec. That can't drive a display

Reply

Answer 2

l0r3nz Author

Level 1

4 points

Mar 29, 2026 11:03 AM in response to Grant Bennet-Alder

I believe the original is a 27MD5KL-B or similar variant I need to track down.

Both monitors are using LG Thunderbolt 3 2m 40gbps 100W cables, plugged directly to the back usb C ports of the studio.

I have tested both monitors with my laptop and though the 27MD5KL registers and continues the display, the LG 27UD88 has a persistent no signal. I need to test additional cables, but ran perfectly for a week before this and the only clear change was getting the Mac to sleep for a period of time. But it is beginning to seem like an LG not apple issue.

I have tested with an HDMI and it's a bit wonky, but functional

Reply

Answer 3

Mar 29, 2026 10:40 AM in response to l0r3nz

what model LG Ultrafine is the original display?

what cables, adapters, and docks are being used to make BOTH connections?

These displays are running very close to the maximum switching speeds for the type of digital logic in use, so cable type and cable length suddenly can become an issue when it never was before.

Reply

Answer 4

Mar 29, 2026 11:11 AM in response to l0r3nz

that original 27MD5KL-B appear to be a full 5K display at 10 bits/color (HDR) capable of refresh rates 48 to 60 Hz.

interfaces include:

ThunderBolt-3 with up to 94 W power delivery.

However:

Ordinary cables you need for running a display at that speed are generally limited to 0.5 meters max. Cables "shipped in the box" (except with Apple displays) are often "lowest bidder" cables, and are not good enough for trouble-free operation.

Genuine Thunderbolt cables must have the Thunderbolt logo (thunderbolt with arrow at end) else they are simply USB cables.

There are a few fully qualified slightly longer cables available, but to get to TWO meter length, you would need an ACTIVE cable with expensive signal re-drivers in it (starts at about US$100 and up).

Reply

Answer 5

Mar 29, 2026 11:28 AM in response to l0r3nz

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

DIRECT HDMI uses Higher Voltages for its signals. This allows for small-room length cables without repeaters, but it precludes using adapters from the Mac TO HDMI.

Reply