M4 MacBook Pro 14" only outputs to one external display at a time (Thunderbolt + HDMI), persists after logic board replacement
Hey, I've been fighting this for a while and I'm out of ideas, hoping someone has seen it.
Setup
- MacBook Pro 14" M4, macOS 27.0.1 (same behavior on the previous release, macOS 26, and the betas)
- Monitor 1: Gigabyte Aorus FO32U2P, connected via Thunderbolt/USB-C
- Monitor 2: LG UltraGear 27G810A-B, connected via HDMI
- No docks or hubs in the chain (direct connections)
- Usually used with the lid closed, but the issue is identical with the lid open
Both of these are officially within what the M4 MacBook Pro should support (two externals).
Problem
Only one external display works at a time. Whichever one is connected first wins:
- Monitor A connected, picture is fine.
- Plug in monitor B while A is connected: B gets no signal, A keeps working.
- Unplug A, B immediately gets picture.
- Plug A back in while B is connected: A gets no signal.
- Unplug B, picture moves to A.
The order doesn't matter, the second monitor never shows up. The second display is not listed at all in System Settings > Displays, not even as a disconnected/black display. It's a detection problem, not "detected but black".
What I've tried
- Different HDMI cable
- HDMI to Thunderbolt/USB-C converter on the LG
- HDMI hub
- Lowered refresh rates on both monitors
- Latest macOS, previous macOS (26), and beta builds
- Full reset of the Mac
- Tested with the lid open as well, same behavior
- Logic board replacement at the service center. They said the testing was completed, but couldn't give me a real explanation for the original fault. Problem is identical after the swap.
Both monitors work perfectly together on my Windows PC, so I don't think the monitors or cables are the issue.
Since a new logic board didn't change anything and it reproduces across multiple macOS versions, I'm wondering if this is something about how these specific monitors negotiate with the M4 (both are high-refresh 4K panels, so maybe a DSC/bandwidth allocation thing?), or if there's a known issue I'm missing. Has anyone gotten two high-refresh externals working on an M4 MBP, or seen a "second display is never detected" behavior like this?