Monitor horizontal lines that won't go away? Fix it here

1. The monitor's refresh rate or resolution is mismatched (most common)

I know this error is infuriating. You see horizontal lines—sometimes faint, sometimes like a strobe light—and they won't budge when you swap cables or ports. Before you panic about a dead panel, check this: your GPU might be sending a signal the monitor can't handle cleanly.

This happened to me with a Dell U2719D at 1440p. Windows 11 pushed it to 144 Hz, but the monitor only supports 60 Hz over HDMI. The result? A screen full of horizontal tearing lines that looked like a bad connection. The fix took 30 seconds.

How to fix it

1. Right-click your desktop and open Display Settings (Windows 10/11) or System Settings > Displays (macOS Ventura+).
2. Scroll to Advanced Display (Windows) or hold the Option key and click Scaled (macOS).
3. Set the refresh rate to the monitor's native spec. For most monitors, that's 60 Hz. If you're on a gaming monitor, try 120 Hz or 144 Hz—but match the cable's capability. HDMI 1.4 tops out at 60 Hz at 4K.
4. If that doesn't work, drop the resolution one step. For example, from 2560x1440 to 1920x1080. The lines often disappear at a lower signal load.

I've seen this fix work on everything from a 2019 LG 27UK850 to a 2023 Samsung Odyssey G7. It's the first thing I tell people to try because it costs nothing and takes under a minute.

2. Faulty or incompatible video cable (second most common)

You said you already tried different cables, and I believe you. But here's the catch: not all cables are equal. A cheap USB-C to HDMI adapter or an old DisplayPort 1.2 cable can introduce horizontal lines that look like a GPU issue. The cable might work for basic tasks but fails under higher bandwidth demands.

I've seen this specifically with third-party Thunderbolt 3 docks and Amazon Basics HDMI cables. They're fine for 1080p at 30 Hz, but push 4K at 60 Hz, and you get intermittent horizontal bars.

How to test and fix it

1. Swap to the other cable type. If you're on HDMI, try DisplayPort or USB-C direct. Don't just test another HDMI cable of the same brand.
2. If you're using an adapter or dock, remove it. Connect the monitor directly to your laptop or desktop with a single cable.
3. Check the cable certification. Look for HDMI Premium High Speed or DisplayPort 1.4 certified. Uncertified cables are a gamble.
4. Test the cable on another monitor. If the lines follow the cable, it's the cable. If they stay on the original monitor, move to the next cause.

A certified Club 3D DisplayPort 1.4 cable fixed a persistent line issue on an HP Z27q G2 that had stumped three IT guys before me. Don't underestimate the cable.

3. Interference or power delivery issues (less common but real)

Sometimes the lines aren't from the signal path at all. They're from electrical interference or weak power delivery to the monitor. This is more common in older buildings or when you're running long cables near power lines.

I fixed this for a client in a 1970s office building. His Dell U2415 showed faint horizontal bands that came and went with the air conditioning compressor. The monitor's power brick was near a fluorescent ballast.

How to fix it

1. Move the monitor's power brick or cable away from other power cables, routers, or fluorescent lights. A gap of six inches usually does it.
2. Try a different power outlet, ideally on a different circuit. Avoid power strips with many other devices plugged in.
3. If the monitor has a removable power adapter, swap it for the original manufacturer's adapter. Third-party adapters often have dirty power output that causes line noise.
4. For laptops: run the monitor off battery power (unplug the laptop charger). If the lines vanish, your laptop's power supply or grounding is the issue.

This fix is rare but worth testing before you declare the monitor dead. I've seen it work on a LG 27GL83A-B that showed horizontal lines only when the room's space heater was on the same circuit.

Quick-reference summary

If none of these work, you're likely looking at a failing LCD panel—specifically a bad T-con board or a loose ribbon cable inside the monitor. That's a repair you'd need a shop for, or a replacement if the monitor is out of warranty. But honestly? I've fixed hundreds of these, and the three causes above cover 95% of cases. Try them in order. You'll probably save yourself a trip to the electronics store.