External screen not connected. "Accessory partially enabled"

Monitor manufacturers sometimes put “cheapest bidder” cables in the box. Modern versions of macOS are reportedly intolerant of display transmission errors, and may cut signal if they detect errors.

So what you want is a good-quality cable rated for 4K video and for at least the amount of charging power your monitor can deliver. It doesn’t need to be a Thunderbolt one (unless you have a Thunderbolt monitor). Try to keep length down to 1 meter / 3 feet or less, as longer cables may be more prone to transmission errors.

Buy:

**A certified USB-C to USB-C cable that explicitly states:

“Supports DisplayPort Alt Mode”

“Supports 4K video”

“USB 3.1 Gen 1 or Gen 2”

“100W PD”**

Cost: $15–$25

This will restore:

• Video
• Charging
• One-cable connection just like before macOS 26.1

Your monitor will work perfectly.

Your Mac is fine.

Your old cable simply wasn’t a video-capable USB-C cable, and macOS 26 stopped accepting it.

Then it sounds like you cables are under control.

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.

Modern Displays with multiple ports are sometimes busy scanning the other ports, looking for an input, and miss the query from the Mac. They need to pay attention to the port you are actually using, or they will miss the query.

Some displays have On-Screen Display settings that can be used to tell the display a computer is attached on a certain port, or a certain port should be highest priority. Changing those may make your display more responsive.

teemu114 wrote:

• Bought a cable with these specifications. Still the exact same message. The cable box says:

4k resolution

• 5 A 100 W (5 A x 20 V PD 3.0) 240 W (5 A x 48 V PD 3.1)

• USB 3.1 SuperSpeed+, up to 10 Gbps.

I've been trying to go to 4 different stores, in none of them any of the sales staff knows what "DisplayPort-capable" cable means, and then they try to sell me DisplayPort-USB-C cables.

Could you please be a dear, and post some actual links to cables that will work for sure, because I'm losing my mind with this.

Since you did not say, I need to ask again:

¿Is this cable ONE meter or shorter?

Longer cables are too lossy for this application under the most recent macOS, which has had an increasingly demanding requirement that display data be received at the display with no transmission errors whatsoever.

I don’t know if this might apply.

Allow USB and other accessories to connect to your Mac - Apple Support

My only Apple Silicon Mac is a desktop one that doesn’t require approval of accessories.

I cannot post links to cables that will work “for sure”, as I have no way of experimenting with your setup and finding out what is wrong.

However, nothing sounds wrong with the specifications of the cable you mention.