I have a MacBook Pro (M4) connected to an external monitor via a USB-C to USB-C cable. It was working fine until I upgraded the OS to macOS 26 earlier today. However, the external monitor is no longer detected, even after I restarted the MacBook Pro.
MacBook Pro 14″, macOS 26.0
Make sure the monitor is powered on before connecting it to the Mac. FWIW, I have a 16" M4 Pro MacBook Pro and I'm typing this on an external display, although mine is connected via Thunderbolt (TB cable to a Stone Pro TB3 dock then a TB cable to TB3-capable LG 5K:2K display).
hlsu wrote:
I have a MacBook Pro (M4) connected to an external monitor via a USB-C to USB-C cable. It was working fine until I upgraded the OS to macOS 26 earlier today. However, the external monitor is no longer detected, even after I restarted the MacBook Pro.
Nice to see things are working smoothly...
I would shut down and restart more than once to sort glitches on an upgraded macOS.
ref: Connect one or more external displays with your Mac - Apple Support
"External Monitor Not Detected After macOS 26 Upgrade: I have a MacBook Pro (M4) connected to an external monitor via a USB-C to USB-C cable. It was working fine until I upgraded the OS to macOS 26 earlier today. However, the external monitor is no longer detected, even after I restarted the MacBook Pro."
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Verify the Connection:
Just curious: How are you verifying this connection? To, see if the monitor is even detected,...
Hello, I may be experiencing a similar issue. I have used a TV for my display for ages and after updating to macOS 26 yesterday, the screen is blank. When I plug the same exact cord into a computer monitor, it displays properly. So maybe there is an issue relating to using a TV as a display?
My computer is a Mac Studio M1 Max, my TV is a Vizio 65", cables are HDMI [regular?], and I am going through an Onkyo audio receiver.
As with the OP, this same setup was working fine before upgrading yesterday, and still works fine when plugged into a computer monitor instead of my TV.
that Samsung CH890 display is a 3440 by 1440 display, like a 4K display with the bottom half missing. it supports 8 bits/color and refresh rates up to 100 Hz.
interfaces include:
2x HDMI 2.0
1x displayport 1.2
1x USB-C providing up to 65W power
to connect via USB-C, your USB-C cable must be certified and show the USB-SuperSpeed PLUS logo or the USB-20 logo, and be ONE meter or shorter. longer cables could cause transmission errors, which would cause the display to drop out.
over DisplayPort or USB-C, provided the cables are adequate spec and not too long, standard 4K 8 bits/color displays support up to 75 Hz without compression, and likely higher with Display Stream Compression.
To get the attention you deserve, PLEASE start a NEW discussion.
If you did not immediately find your solution by reading through the existing postings, you likely do not have "exactly the same problem" and need to explain from the beginning. This discussion is already too complicated.
Starting a new discussion will allow Readers to think hard about your exact situation and address the exact circumstances you are facing.
https://discussions.apple.com/post/question
The external monitor was not shown up in the system information (Graphics/Displays).
[Edited by Moderator]
Before the upgrade to macOS 26, the external monitor was connected (via USB-C) as an extended display and it was working fine.
The Mac does not rely on Windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.
To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.
This query is only sent at certain times:
• at startup
• at wake from sleep — so momentarily sleeping and waking your Mac may work
• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go
• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)
so try doing some of those things and see if the display comes alive.
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.
what Processor type? {M-series (plain), PRO, MAX, ULTRA}
what make&model display?
What cables, adapters and docks are being used to make the connection?
It's M4 Pro. The monitor is a Samsung 34" (CH890), connected via a USB-C to USB-C cable, no adapter or dock.
The external monitor was detected and working fine before upgrading to macOS 26.
For now I have to use a separate HDMI cable to connect to the external monitor, which was detected properly.
[Edited by Moderator]
External Monitor Not Detected After macOS 26 Upgrade
