Screens · guide

Half black screen, and the test that removes the computer

By Alberto Gulotta · Updated 18 September 2026 · 18 min read

A half black screen is not one fault with one answer. It is a question about which side of the machine is broken, and every screen has a test built into it that settles that question without a shop and without a cable. The quickest of them runs with the computer unplugged altogether.

The reason this is worth saying first is that the ordinary advice for this symptom is written for one case out of four. Restarting the graphics driver, unplugging the power for ten minutes, checking that the cables are seated: every one of those is a test of the path between a computer and a screen. They are good moves, and they are wasted on a panel that is damaged, which is why the order matters more than the list. Two of the four results below make all of them unnecessary.

1. Work out which of the three tests is yours. A laptop screen, a desktop monitor and a phone each have a different built-in test, and they are not interchangeable. Dell puts the boundary plainly in its own FAQ: asked whether the laptop test works on a desktop monitor, the answer is “No, this test is specifically for Dell laptop screens.” The table below sorts them.
2. On a monitor, take the video cable out and leave the power in. That is the self-test feature check. Dell describes exactly the case it is for: “If the Dell monitor is connected to a computer, but the monitor screen remains dark or it shows a blank screen, the self-test feature check (STFC) helps verify if the Dell monitor is functioning correctly.” A working monitor with no signal shows a floating box on a black background; a half that stays dark behind that box is the monitor’s own.
3. On a laptop, run the panel test from cold. On a Dell, hold D and press the power button. It starts before Windows loads and, in Dell’s words, “shows solid colors that fill the entire screen. The colors appear in this order: black, white, red, green, and blue.” The dark half either survives those colours or it does not, and there is nothing in between. The key presses for other makers are in a vertical black line on a laptop screen.
4. Only now try the software moves — and listen rather than look. Microsoft documents the shortcut everybody repeats and one detail almost nobody passes on: “Press Windows + Ctrl + Shift + B to reset the graphics driver. You should hear a beep or see a screen flicker if successful.” On a screen that is half dark the beep is the useful half of that sentence, because it tells you the machine heard you. Windows + P is the other one: Microsoft has you tap P again and press Enter to cycle through the display modes.
5. Plug in a second screen and look at which of the two is wrong. If the outside screen is perfect and the built-in one is still half dark, the machine is making a whole picture and losing half of it on the way to its own screen; if the panel test in step 3 showed the dark half too, that is the display assembly. HP reaches the same fork from the other direction: for a picture outside and nothing inside its next step is the latest BIOS, graphics driver and chipset, and if nothing appears on either screen, the question is no longer the panel at all, it is the machine starting up.
6. Photograph the boundary, with the date. Whether the dark area is growing is the one fact that changes what to do next, and it is a comparison rather than a memory. It is also the first thing a repairer will ask.

The three tests are not the same test on different hardware, and the difference is the whole reason for the table. The laptop panel test runs from the machine’s own firmware before Windows loads, so it takes the operating system out of the picture but keeps the laptop’s own electronics in it. The monitor’s self-test feature check goes further and takes the computer out entirely. The phone has neither: the nearest thing is safe mode, which Samsung describes as a way “to determine if the issue is caused by an app or with the device itself” — an app, not a panel. So on a phone the evidence has to come from somewhere else, and a blue spot on a phone screen uses a screenshot for it.

There is one more reading worth making before spending anything, and it is free. If part of the screen still shows the picture correctly — right colours, right shapes, the pointer moving through it — then the machine is producing a good image and sending it out. Whatever is wrong is downstream of that. That reasoning is ours rather than any manufacturer’s, and it is set out at length on half a laptop screen gone white, which is the same argument on the other colour. Where a half-dark screen differs is that black is also what a screen shows when it is doing nothing at all, so the test matters more here: a white half is always the panel, a black half might be a panel that is fine and receiving nothing.

Two neighbouring cases, in case this is not quite yours. If the whole screen is dark rather than half of it, and you can hear the machine running, the question is whether the picture is being drawn at all — that is a display backlight that stops working, and it starts somewhere else because the built-in test cannot help when you cannot see what it draws. And if the second screen is the one that has gone dark, it may never have been seen in the first place: a second monitor that is not detected is a different failure with different steps.

Not covered here. It will not name the broken component. The six documents read for this page do not name one for this symptom, and a repairer with the machine open can say more than any page.

It will not walk through replacing a panel or a backlight strip. Where that is the answer, it is the answer after the test, not instead of it.

It will not price the repair. None of the six documents read here publishes a figure, and the only number worth having is a quote on your own machine. What holds instead is simple: the sentence that the built-in tests only check the screen hardware and do not check Windows settings, graphics drivers or software, the colour sequence of the laptop panel test, and the statement that it is a diagnostic tool and not a repair tool, are quoted from Dell’s article 000134936; the description of the self-test feature check as a way of checking whether the monitor is working as a stand-alone device, and the case it is for — a monitor connected to a computer whose screen remains dark or blank — are quoted from Dell’s article 000124390; the gesture of the monitor check — unplugging the video cable, plugging the power cable back in and turning the monitor on — and the floating box on a black background that means the monitor is working but has no video signal are read from Dell’s article 000126004; the instruction to press Windows, Ctrl, Shift and B to reset the graphics driver, with the note that you should hear a beep or see a screen flicker if it worked, and the Windows and P route through the display modes, are quoted from Microsoft’s page on troubleshooting blank screens in Windows; the fork between a fault in the display panel and a fault starting the computer, and the next step when an image appears on the external monitor but not on the computer — the latest BIOS, graphics driver and CPU chipset — are read from HP’s document ish_7096250-7096363-16; and the description of safe mode as a way of determining whether an issue is caused by an app or with the device itself is quoted from Samsung’s page on pink lines or dead pixels.