The HP laptop with a black screen that wasn't actually dead

A coworker's HP came in with no display and a dead keyboard and trackpad. A flashlight showed the screen was still drawing an image. Here's the diagnosis so far.

A coworker handed me their HP laptop with two problems: the screen stayed black, and the keyboard and trackpad didn’t respond. Most people would call that a dead laptop. It isn’t, at least not yet.

The flashlight test

When a laptop screen goes black but the machine seems to be on (fans spinning, power light on, maybe a startup sound), the first thing to check is whether the panel is dead or just the backlight.

Turn the laptop on, shine a flashlight at the screen at an angle, and look closely. If you can faintly see the desktop or a login screen, the LCD is still drawing an image. Only the light behind it has gone out.

That’s what I saw here. The image was there, dim and ghostly, which narrows the problem down a lot.

What a dead backlight can mean

On most modern laptops the backlight is a strip of LEDs inside the panel, powered and controlled by the motherboard through the display cable. A dead backlight with a working image usually comes down to one of these:

The LED strip in the panel has failed. Fix: replace the panel.
The display cable is damaged where it runs through the hinge. Cheaper fix.
A backlight fuse or power circuit on the motherboard has blown. This is where the multimeter comes in.

Why the keyboard matters

The keyboard and trackpad not working is the interesting part. It could be a separate problem, like a liquid spill or a loose ribbon cable. Or it could point to something shared on the motherboard, such as a power rail that feeds several parts at once. I don’t want to buy a new screen until I know which.

Next steps

Plug in an external monitor to confirm the laptop itself boots normally.
Open it up and reseat the display, keyboard and trackpad cables.
Use a multimeter on the backlight fuse and the backlight voltage at the display connector.
Order parts only once the cause is confirmed.

I’ll update this post as I go. If you have an HP with the same symptoms, try the flashlight test first. It takes ten seconds and tells you a lot.