Screens · guide

Display backlight stops working completely: where to start

By Alberto Gulotta · Updated 18 September 2026 · 17 min read

When the display backlight stops working completely the picture is usually still being drawn: you are looking at a working screen in the dark. That changes which test is worth running, because the built-in one paints colours you cannot see. Four other outputs can still be read, and they cost nothing.

Before any of those four, two settings deserve a minute, because each of them produces this exact symptom on a machine with nothing wrong, and one of them is not visible from inside Windows at all. Three of the five results written about this fault go straight to opening the screen; neither of these two settings appears in any of them, and both are documented by the manufacturer.

1. Plug the charger in and hold the brightness key down. This is not beneath the problem: Dell documents the behaviour that produces it. “On AC power, your laptop screen is automatically set to full brightness. On battery power, it’s set to reduced brightness to conserve energy.” Press Fn and the brightness key together and keep pressing. Ten seconds, and it clears a whole family of false alarms.
2. Check the ambient light sensor, which is not in Windows. Dell puts the switch in the firmware: turn the laptop off, turn it on and press F2 repeatedly until the BIOS screen appears, then look for a setting called Ambient Light Sensor or ALS and set it to Disabled. A sensor reading a dark room can take a screen down to where it looks dead, and nothing in Settings will show you that it is happening.
3. Press Windows + Ctrl + Shift + B and listen. Microsoft documents both halves of this and the second is the one that matters on a dark screen: “Press Windows + Ctrl + Shift + B to reset the graphics driver. You should hear a beep or see a screen flicker if successful.” A beep means the machine is awake and running and heard you, which on a screen showing nothing is a great deal to learn in one second.
4. Look at the LED, not at the screen, while the panel test runs. On a Dell the enhanced version of the panel test reports through the battery or power light as well as through the picture, and the codes are counted in two colours: “count both the amber (orange) flashes and the white flashes.” Dell lists four new codes, from four amber and three white up to four amber and six white, and notes that they replace an older pair. This is the one output of the test that a dead backlight cannot hide.
5. Plug in an external monitor and read the fork. HP states the question this settles better than anyone: “you must determine if this blank screen is caused by a problem with the display panel or a problem starting the computer.” For a picture outside and nothing inside, HP’s next step is not the panel: “download and install the latest BIOS, the latest graphics driver, and the latest CPU chipset.” Nothing on either, with the machine not starting properly, means you are troubleshooting a computer rather than a screen.
6. On a desktop monitor, take the video cable out and leave the power in. A working monitor with no signal draws its own message. Dell describes what you should see: “a floating box on a black background. This means your monitor is working but has no video signal.” No box and no light is the one result that does not separate anything, and the next section says why.

The LED step is the one worth reading twice, because it is the only place where the maker’s own test is designed to report without using the screen. On a Dell laptop with a current BIOS, the enhanced panel test flashes the battery or power light in a counted pattern of amber and white, and Dell publishes the four codes it can produce. That is a diagnostic channel that does not care whether there is any light behind the panel, which is precisely what this fault needs. It is also the reason to write the pattern down rather than remember it: Dell’s own advice when a test fails is to note what you saw, count the flashes, and photograph the machine before contacting support.

One case is worth separating out, because it is the commonest wrong turn on this symptom. A screen that is dark but has a faint picture visible when you tilt it towards a window, or hold a torch close to it, is a different thing from a screen that shows nothing under any light: the first has a panel still drawing and no illumination, the second may be receiving nothing to draw. That distinction is a reasonable one to make and it is ours — none of the six documents read for this page on 15 September 2026 describes the torch, and it is worth knowing that it comes from workshop practice rather than from documentation. What the documents do give you is the same fork from the other side, and it is less final than it sounds: if an external monitor shows a picture and the laptop screen does not, the machine is producing one, and HP’s next step for that result is firmware and drivers, not the panel.

Two neighbouring cases. If part of the screen is dark and part of it is not, the evidence is better than it is here and the route is different: a half black screen starts from which built-in test is yours. And if the machine itself is not starting — no fans, no lights, nothing — the screen is not the question at all, and a computer that will not boot is the place to begin.

Not covered here. It will not walk through replacing a backlight strip or an inverter. Where that is the answer, it is the answer after the fork rather than instead of it.

It will not name the failed part. None of the six documents read for this page distinguishes a failed backlight from a failed panel, board or cable for this symptom.

It will not price the repair. None of the six publishes a figure, and a quote on your own machine set against what the machine is worth is the only comparison that decides anything. What holds instead is simple: the statement that a laptop screen is automatically set to full brightness on AC power and to reduced brightness on battery to conserve energy, the instruction to press Fn and the brightness key together, and the route to the ambient light sensor through the BIOS with F2 and the setting named Ambient Light Sensor or ALS, are quoted from Dell’s article 000137814; the instruction to count both the amber and the white flashes of the battery or power LED during the enhanced test, the four new flash codes, the note that the enhanced test requires an updated BIOS and that an external monitor is needed to perform that update if nothing can be seen on the laptop screen, and the statement that the tests only check the screen hardware and do not check Windows settings, graphics drivers or software, are quoted from Dell’s article 000134936; the description of the floating box on a black background as a monitor working with no video signal is quoted from Dell’s article 000126004; the case the self-test feature check is for, a monitor whose screen remains dark or shows a blank screen, is read from Dell’s article 000124390; 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, is quoted from Microsoft’s page on troubleshooting blank screens in Windows; and the statement that you must determine whether a blank screen is caused by a problem with the display panel or a problem starting the computer, with the next step when an image appears on the external monitor but not on the computer — download and install the latest BIOS, the latest graphics driver and the latest CPU chipset — are quoted from HP’s document ish_7096250-7096363-16.