Something is wrong
Monitor flickering or cutting out
A monitor that blinks to black for a second, flickers under load or drops signal at random has three possible culprits, and they cost wildly different amounts to replace. Most people replace the expensive one. The order below rules out the cable first because it is the most common cause and the cheapest to swap, and leaves the monitor for last because a panel fault looks like nothing else once you know what to look for.
The run, in order
Notice when it happens, before changing anything
The pattern is half the diagnosis. Flicker only in games or under load points at the graphics card or its power. Dropouts when you move the desk or touch the cable point at the cable. A blink on waking from sleep is usually the link renegotiating and often harmless. Constant fine flicker at low brightness is the backlight, and not a fault at all. Write down which one you have — the steps below mean different things depending on it.
Measure the refresh rate the display is actually running at
A monitor set to 144 Hz over a cable that can only carry 120 will drop frames, blink and sometimes lose signal entirely, and the settings page will still say 144. The measurement shows what is really arriving. A number well below the setting, or one that wanders, is a link that cannot keep up.
Look at the colour-detail pattern
The same bandwidth-starved link that drops frames also drops colour detail. If the red-and-blue stripes are a smear while the black-and-white ones beside them are crisp, the connection has fallen back to a reduced mode — which is the cable, the port or the setting, and never the panel. Two tests agreeing on the link is as close to certainty as this gets without swapping parts.
Run the flicker test at low brightness, then at full
This separates two things that share a name. Backlight flicker is steady, appears at low brightness, disappears at full, and is the panel dimming itself — unpleasant for some people and not a defect. Signal flicker is irregular, does not care about brightness, and is what the rest of this page is about. If the flicker test shows the first kind and nothing else is wrong, you are done, and the fix is a brightness setting.
Swap the cable, and prefer DisplayPort
The cheapest test on this page and the one most likely to end it. Cables fail internally, cheap HDMI cables cannot carry what their packaging claims, and a cable that was fine at 60 Hz is often marginal at 144. Borrow one if you can — the point is a known-good cable, not a new one. If the fault stops, the diagnosis is finished for the price of a cable.
VESA-certified DisplayPort cables on AmazonPaid link: we earn a commission if you buy, at no extra cost to you. As an Amazon Associate we earn from qualifying purchases. Certified is the word that matters — it is what separates a cable that carries its rated bandwidth from one that only prints it on the bag. A borrowed cable tests the same thing for nothing.
Move to another port, then another computer
Two swaps that between them isolate the graphics card. A different output on the same card rules out a single damaged port. A different computer — a laptop will do — rules out the card entirely: if the monitor is stable on the laptop through the same cable, the card or its driver is the fault. If it flickers on both, it is not the card.
Fill the screen with solid colours and look for lines and bands
A panel fault has a signature a cable fault never has: vertical or horizontal lines, a band of wrong colour, or a region that flickers while the rest is still. Solid fields make those impossible to miss. If the screen shows lines on every source, on every cable, the fault is inside the monitor, and it is a repair or a return rather than anything you can fix from the desk.
Drop the refresh rate, temporarily, and watch
Not a fix — a confirmation. If the monitor is perfectly stable at 60 Hz and flickers at 144, the link is marginal and a better cable or a shorter one will fix it properly. If it flickers at 60 Hz as well, the refresh rate was never the problem, and what remains is the card or the panel, which the previous two steps have already sorted between them.
Why the order matters
The three suspects cost very different amounts. A cable is a few pounds; a monitor is a return or a repair; a graphics card is the most expensive part in most computers. The order here spends the cheap tests first, so that the expensive conclusion is only reached once the cheap ones have failed to explain it.
It also starts with two measurements rather than a swap, because a measurement costs nothing and can end the search on its own. A refresh rate that is not what the settings claim, and a colour-detail pattern that has turned to smear, are both the link running out of bandwidth — and two tests pointing the same way is as good as a swapped cable, before a cable has been touched.
Reading the outcome
The cable. Fault stops with a different cable; measured refresh rate below the setting; colour detail smeared; stable at 60 Hz and not at 144. Any two of these is a cable. Buy a certified one — DisplayPort where the monitor has it — and buy it as short as the desk allows.
The graphics card, or its driver. Fault follows the computer to a different monitor, or vanishes when the monitor is connected to a different computer. Flicker only under load points at the card’s power; flicker after a driver update points at the driver, and rolling back is the test.
The monitor. Lines or bands in fixed positions on solid fields; a region that flickers while the rest is still; the fault present on every cable and every computer. This is the panel or its driver board, and it is a warranty claim if there is a warranty. Name the fault precisely in the claim — “vertical line at approximately one third from the left, present on all inputs” is a claim; “the screen flickers” is a conversation.
A setting. Steady flicker at low brightness that disappears at full is the backlight dimming itself. It is not a fault, some people are sensitive to it, and the flicker test explains what to do about it.
The one that is not this fault at all
A blink when the computer wakes from sleep, or when a game switches to full screen, is the link renegotiating its mode. It happens once, the picture returns, and nothing is wrong. If that is the only time the screen goes black, there is nothing here to fix — though a cable swap sometimes makes the renegotiation quicker.
Common questions
Can a cable really cause a monitor to go black?
It is the most common cause there is. The signal to a modern monitor is a high-speed digital stream, and a cable that cannot carry it does not produce a slightly worse picture — it loses synchronisation, the monitor shows black while it re-locks, and the picture comes back a second or two later. That blink-and-return is the classic symptom. Long cables, cheap cables and cables that were sold for 1080p and pressed into 4K duty all do it.
Why does it only flicker at 144 Hz?
Because that is when the link is working hardest. A higher refresh rate is more data per second down the same cable, and a cable that has just enough margin at 60 Hz has none at 144. The monitor is fine, the card is fine, and the cable is the weakest part. A certified DisplayPort cable, or a shorter one, is the fix — not a lower refresh rate.
The screen has a vertical line that never moves. Cable or monitor?
Monitor. A line that stays in the same place regardless of what is on screen, survives a cable swap and appears on a different computer is a fault in the panel or the board that drives it. Nothing outside the monitor can draw a line in a fixed position. It is a return if the monitor is under warranty and usually not worth repairing if it is not.
It goes black for two seconds and comes back. Is that dangerous?
Not to the hardware, and not a sign it is about to die. It is the link losing sync and re-establishing it, and while it is irritating it does no damage. It does mean something is marginal — nearly always the cable or a port — and the steps above will find which.
My laptop screen flickers. Does this apply?
Partly. A laptop screen has no external cable to swap, but it does have an internal one that runs through the hinge, and it wears — a screen that flickers when the lid is moved is that cable, and it is a known and repairable fault. Flicker at low brightness only is the backlight dimming and is normal. Flicker under load with the lid still is the graphics driver, and updating or rolling it back is the first thing to try.
Could it be the power supply?
On the graphics card side, yes: a card that flickers or blacks out only under heavy load, and is fine on the desktop, may be drawing more power than the supply can deliver cleanly. That is a computer fault rather than a display one, and the tell is that it follows load rather than following the cable or the monitor. On the monitor side, an external power brick that is failing produces flicker that follows the monitor to another computer — which the swap step will show.