I bought a new USB-C to DP cable for my second monitor, plugged it in, and got "No DP signal from your device". Obviously the cable was broken. I changed ports, flipped the plug, restarted the Mac, and got angrier with every try.
The cable was fine. It was my fault, and the order I plugged my monitors in. Here's what actually happens, and how to fix it in 10 seconds.
Tested on 6 October 2026 with an M5 MacBook Pro, a Philips 27M2N3800A (4K @ 160Hz, HDMI) and a Dell S2721HGF (1080p @ 144Hz, USB-C → DP).
Short version: if one high-refresh monitor works and the second one shows "No DP signal", unplug the working monitor, wait for the second one to light up, then plug the first one back in.
Symptoms
- Monitor A works fine. Monitor B stays black and shows "No DP signal from your device".
- Monitor B is detected at a low level, because macOS reads its name and specs, but it never shows up in System Settings → Displays.
- Changing ports, flipping the USB-C plug, replugging, Detect Displays, display sleep and even a full restart change nothing.
- It looks exactly like a broken cable, but the cable is fine.
The fix
If your MacBook lid is closed, open it a little first. Otherwise unplugging monitor A leaves zero active screens and the Mac goes to sleep (clamshell mode needs an active display).
Leave monitor B (the dead one) plugged in and unplug monitor A's cable from the Mac. Wait about 10 seconds, and monitor B lights up. Then plug monitor A back in. Both now work.
If monitor B comes up at a weird low resolution like 800×600, it is mirroring. Go to System Settings → Displays, set it to "Extend", and pick its native resolution.
To avoid it in future: plug in the lower-spec monitor first, then the high-refresh 4K one.
Why it happens
The Mac has a fixed number of internal display pipes (video outputs) for external monitors. The M5 has two.
A demanding monitor such as 4K at 160Hz can take both pipes if it connects while nothing else is plugged in. The second monitor is then detected but left with no pipe, so the Mac never sends it a picture. A restart doesn't help, because the same monitor grabs both pipes again on boot.
When the second monitor connects first, it takes one pipe and the 4K monitor settles for the other. One pipe is still enough for 4K at 160Hz.
| Plug-in order | Result |
|---|---|
| 4K @ 160Hz first | It takes both pipes, and the second monitor gets "No DP signal" |
| Second monitor first, then the 4K | One pipe each, and both work |
Confirm it in Terminal
ioreg -lw0 | grep -oE '"ConnectionMapping" = \(\{[^)]*\)[^)]*\)' \
| grep -oE '"ProductName"="[^"]*"|"role"="[^"]*"|"PipeIDs"=\([^)]*\)|"AutoConnect"=(Yes|No)'
- Broken: the working monitor shows
"PipeIDs"=(1,2), and the dead one shows"role"=""and"AutoConnect"=No. - Fixed: each monitor has one pipe,
(1)and(2), plus a role (DCPEXT0/DCPEXT1).
What doesn't help
| Tried | Why it doesn't work |
|---|---|
| Restart | The same monitor grabs both pipes again on boot |
| Other USB-C ports / flipping the plug | The port and cable aren't the problem |
| Detect Displays (hold Option in Displays settings) | macOS sees the monitor but has no pipe to give it |
| Sleep/wake displays | Same reason |
| Lowering the first monitor's refresh rate | It keeps both pipes until it's reconnected |
| Buying or returning cables | The cable was fine the whole time |