You plug the cable in, the monitor says “No Signal”, and the MacBook acts like nothing happened. Frustrating, and usually fixable in under five minutes once you know which of the handful of causes you’ve hit.
Here they are in the order worth trying, roughly most common first.
First, the one thing you need to understand
USB-C is a connector shape, not a guarantee of what’s inside. Video over USB-C only works if the port supports DisplayPort Alternate Mode (DP Alt Mode), where the port reroutes some of its high-speed lanes to carry a DisplayPort signal. A USB-C to HDMI cable then converts that DisplayPort signal to HDMI inside the connector.
Good news for Mac owners: every USB-C and Thunderbolt port on a modern MacBook supports it. So on a Mac, DP Alt Mode is almost never the fault — the cable, the display settings or the chip’s display limit usually is. (On cheap Windows laptops and many Android phones, it genuinely can be the port.)
Fix 1: Check the cable is a video cable
This is the single most common cause. A USB-C cable that came with a phone, a power bank or a set of earbuds is a charge-and-data cable. It has no idea what DisplayPort is, and it will never carry video, no matter how expensive it was.
You need either a purpose-built USB-C to HDMI cable (the conversion electronics are in the plug) or a USB-C hub with an HDMI output. A plain USB-C cable plus an HDMI coupler will not work.
Fix 2: Change the input source on the monitor
Obvious, and still the answer maybe one time in five. Monitors and TVs with several HDMI ports don’t always auto-switch, especially if another input has an active device on it. Cycle through the inputs manually with the monitor’s own buttons or remote, and confirm you’re on the exact port you plugged into — HDMI 1 versus HDMI 2 matters.
While you’re in there: if the monitor has an “HDMI 2.0” or “Enhanced/UHD” toggle per port, turn it on. Many displays ship with ports limited to HDMI 1.4 mode by default, which caps you at 4K30 or produces no picture at all at higher settings.
Fix 3: Force macOS to detect the display
Sometimes the Mac just misses the handshake. In System Settings → Displays, hold the Option key and a Detect Displays button appears — click it.
If that does nothing, in order:
- Unplug the cable, close the lid to sleep the Mac, wait ten seconds, open it, plug the cable back in.
- Restart the MacBook with the cable connected.
- Try the other USB-C port. Ports do fail individually, and this tells you instantly whether it’s the port or the cable.
Fix 4: You may have hit your chip’s display limit
This one catches a lot of people and no amount of cable swapping will fix it. Apple silicon caps how many external displays a chip can drive, and the base chips are the strictest.
Base M1 and M2 MacBook Air and 13-inch Pro models support one external display. Later Air models added support for a second display with the laptop’s lid closed, and Pro and Max chips support more. Check Apple’s tech specs page for your exact model before you spend anything.
If you’re stuck at one display and need two, your options are a DisplayLink-based dock (which uses software compression rather than DP Alt Mode) or a machine with a Pro/Max chip. A second cable or a bigger hub won’t get you there.
Fix 5: Check what the hub is actually rated for
If you’re going through a hub, its HDMI output has its own ceiling. A lot of budget hubs do 4K at 30Hz only — which looks fine on a spec sheet and feels horrible in use, because the mouse pointer visibly stutters.
4K at 60Hz needs 18Gbps of HDMI bandwidth (HDMI 2.0). Our 7-in-1 USB-C hub outputs HDMI at 4K@60Hz and needs no drivers on macOS, which also avoids the second common hub problem: driver software that never got updated for the current macOS version.
Fix 6: Rule out the HDMI cable and the display end
If your USB-C to HDMI adapter has a separate HDMI cable running to the monitor, that cable is a suspect too — particularly old ones bundled with a TV years ago, which may be HDMI 1.4 (10.2Gbps) and unable to carry 4K60.
A quick isolation test: plug something else known-good into the same monitor and cable — a games console, a set-top box, another laptop. If that also fails, the problem is downstream of your Mac entirely. A current 18Gbps 4K@60Hz HDMI cable removes the variable for under $15.
Fix 7: Drop the resolution or refresh rate
If you get a picture that flickers, drops out every few seconds, or shows sparkles and snow, that’s usually a bandwidth or signal integrity problem rather than a compatibility one. Long runs, cheap cables and 4K60 don’t mix well.
In System Settings → Displays, hold Option and click Scaled to see the full list of modes, then try 4K at 30Hz or 1440p at 60Hz. If the picture becomes rock solid, you’ve confirmed it’s bandwidth — shorten the run or use a better-rated cable.
Quick diagnosis table
| What you’re seeing | Most likely cause | What to do |
|---|---|---|
| Nothing at all, monitor says “No Signal” | Charge-only USB-C cable, or wrong input selected | Use a true USB-C to HDMI cable; cycle monitor inputs |
| Works on one MacBook, not another | Display limit on the base M-series chip | Check Apple’s specs for your model; consider DisplayLink |
| Picture appears, mouse feels laggy | Running at 4K30 | Enable the monitor’s HDMI 2.0 mode; use a 4K60-rated path |
| Flickering, dropouts, sparkles | Bandwidth or cable quality | Shorter or better cable, or lower the refresh rate to test |
| Audio missing but video fine | macOS output device not switched | System Settings → Sound → Output, select the display |
| Works, then drops after sleep | Handshake on wake | Unplug and replug; update macOS; try the other port |
When it’s time to stop troubleshooting
If you’ve tried a known-good video cable, a different port, a different monitor and a restart, and you still get nothing, the hardware has failed and more fiddling is wasted time. Under Australian Consumer Law, a cable or adapter that doesn’t do the job it was sold for is faulty regardless of how long ago you bought it, so go back to the retailer rather than buying a replacement and hoping.
FAQs
Do all USB-C ports support HDMI output?
No. Video over USB-C requires DisplayPort Alt Mode. Every USB-C and Thunderbolt port on a modern MacBook supports it, but many budget Windows laptops, tablets and phones do not.
Can I use a normal USB-C cable with an HDMI adapter?
Only if the adapter is designed for that and the cable carries the high-speed lanes. In practice a single USB-C to HDMI cable is more reliable than a stack of adapters, because there are fewer places for the signal to fail.
Why does my external monitor only show 30Hz?
Something in the chain is limited to HDMI 1.4 bandwidth — usually the monitor’s port setting, an older HDMI cable, or a hub rated for 4K30. 4K at 60Hz needs an 18Gbps HDMI 2.0 path end to end.
Why can’t my M1 MacBook Air run two monitors?
The base M1 chip supports one external display. That’s a limit of the chip, not the cable or the hub. A DisplayLink dock is the usual workaround.
Does resetting the SMC or NVRAM help?
Not on Apple silicon — those resets no longer exist. A normal restart with the display connected does the equivalent job.
Still stuck?
If it turns out you just need a cable that was built for video, ours ship from Australia in 2–5 business days rather than the three weeks an overseas order usually takes, and there’s a 30-day money-back guarantee if it doesn’t solve your particular version of this problem.
