You plug your laptop into a monitor with a USB-C cable. The battery icon shows it's charging, so the connection is obviously working, but the screen stays black. You swap in the cable from your desk drawer and the picture comes right up.
Both cables have the same connector, about the same thickness, and the same plain black jacket. One of them just doesn't have the wires for video.
In our last article we followed a black screen to the middle of a cable chain. This time the cause is usually simpler. It's the cable itself, or the port it's plugged into.
In the last piece we saw that a USB-C connector has 24 pins in two rows. A cable doesn't have to connect all of them, and many cables don't.
The USB-C standard allows two very different cables behind the same plug:
USB-C video, called DisplayPort Alternate Mode, runs on those high-speed pairs, with the DisplayPort AUX channel on the SBU wires. A USB 2.0 cable can negotiate power perfectly over CC, but it can't carry video. The laptop sees no video path and the display has nothing to show. Neither one reports an error.
That's how a cable can "work" (it charges, and your phone even syncs over it) and still never show a picture.
This catches a lot of people. A cable stamped 100W or 240W has heavier power conductors, and usually a small chip in the plug called an e-marker that tells the charger it can safely carry 5 amps. Those cables are very often USB 2.0 inside. The power rating and the data rating are separate specs, and a cable can be top of the line for one and minimal for the other.
So "it's a 100W cable" doesn't tell you whether it will carry video. You need to check the data rating.
Check the speed rating. Look for a data speed on the connector, jacket, or packaging: 5Gbps, 10Gbps, 20Gbps, 40Gbps, 80Gbps, a USB4 label, or a Thunderbolt lightning bolt. Any of these means the cable has high-speed lanes. Newer certified cables carry USB-IF logos that show speed and wattage together. If the only number is a wattage, or the cable says "USB 2.0" or "480Mbps," don't count on it for video.
Don't go by thickness. USB 2.0 cables have fewer conductors and are often thinner and more flexible, but a heavy 240W charging cable can be just as thick as a full-featured one. Treat thickness as a hint at most.
Test the link speed. If you have a USB-C SSD or fast flash drive, connect it directly with the cable in question. Free utilities like USB Device Tree Viewer on Windows, or System Information on a Mac, show the negotiated speed. If you see SuperSpeed (5Gbps or faster), the high-speed lanes are present. A 480Mbps link means a USB 2.0 cable, or a problem somewhere in the path.
Use a cable tester. Inexpensive USB-C cable testers have a receptacle on each end and LEDs that show which conductors are continuous. A full-featured cable lights up the TX/RX and SBU lines. A USB 2.0 cable lights up only power, D+/D−, and CC. If you need to sort a drawer of unlabeled cables, a tester pays for itself quickly.
When you find a cable that only charges, label it or throw it out. An unlabeled charge-only cable will cost someone time again later.
If a known video-capable cable still shows a black screen, check these next.
The port doesn't output video. Not every USB-C port supports DisplayPort Alt Mode. Many budget laptops, some desktop motherboards, and a lot of phones have USB-C ports that do charging and data only, and some laptops support video on just one of their USB-C ports. Look for a small DisplayPort symbol or a Thunderbolt bolt next to the port, or check the spec sheet for "DisplayPort Alt Mode" or "display output." If the port doesn't support video, no cable will fix that.
The display's USB-C port doesn't take video. On some monitors the USB-C port is only for a hub or for charging. Check the manual, and make sure the monitor's input is set to USB-C.
There's a junction in the path. If a coupler or passive extension sits between two cables, the CC wire only connects end to end when the plugs are lined up. That's exactly the problem we tracked down in the first article in this series. If video comes and goes when you flip a plug, start there.
The run is too long. High-speed signals fade over distance much faster than power does. As a rough guide, passive full-featured USB-C cables are reliable for full-speed video and data up to about 1 meter (3.3 ft) at 10–20Gbps and a bit less at 40Gbps. USB 2.0 data and charging hold up to about 4 meters (13 ft). A long cable can charge perfectly and still drop video. For longer runs, use an active cable or extender, or convert to HDMI or DisplayPort at the source.
Changing one thing at a time usually finds the cause within a few minutes.
In conference rooms, classrooms, and hot desks, nobody checks which USB-C cable is on the table. They plug in and expect the screen to work. If a charge-only cable ends up at a desk where people present, you'll get support calls for it.
Use one continuous full-featured cable, rated for DisplayPort Alt Mode, at the length the run actually needs. If the run is too long for a passive cable, use an active extension. Label the cable at the plug end so it doesn't get swapped for the first USB-C cable someone finds in a drawer.
Need a video-rated USB-C cable in a specific length? Browse our [full-featured USB-C cables] or reach out and we'll help you spec the right cable for your run.
Coming up in this series: decoding USB-C speed ratings (what "USB 3.2 Gen 2x2" actually means) and what an e-marked cable is and when you need one.