Screens · guides

Computer screen problems, and which half of the machine is at fault

By Alberto Gulotta · Updated · 7 min read

This is the entrance to the screens tower — the guides for a screen that has started doing something it did not do last week. A line down the picture. A pale patch spreading out of one corner. A shiver you notice out of the side of your eye. A touch that goes unanswered. Or nothing at all, on a machine you can hear running.

Every one of these computer screen problems splits in the same place, and the split is worth more than any single repair. Either the panel in front of you is damaged — the glass, the layer behind it that makes the picture, the flat ribbon that carries the picture into it — or the panel is perfectly good and something behind it is feeding it the wrong thing: a cable, a driver, a setting, a board. From where you are sitting the two look identical. In money they are nowhere near each other. So every guide here starts by telling them apart, with a check you can make in a minute and without buying anything, and only then talks about repair.

The second thing this tower does is rarer, and it is most of the reason it exists: it says when to stop. The screen is the most expensive single part of most laptops and most phones, and a great deal of what is written about these faults is published by the people who sell the replacement panel. That is why so much of it ends at “book a repair” and none of it ends at “this one is not worth mending”. Every guide here carries the second half — what the part costs set against what the machine is worth, and the point where the sensible answer is to stop spending on it.

One thing gets said once here rather than on every floor. Some of these faults sit still for years and some spread, and the difference is not cosmetic: it decides whether this is a repair to plan for or a repair to make this week. A mark left by pressure and a crack in the layer behind the glass can look the same on the day they appear and behave completely differently afterwards. Each guide says which of the two it is describing, and what it is based on.

Three groups below, and they are in the order you want them. The first separates the two halves of the machine. The second is the marks on the glass themselves, one shape of mark at a time. The third is a picture that will not stay still, or a screen that will not answer at all.

Where to start

Five ways in. If the screen is blank rather than marked, take the first one: it is a different fault with different steps.

“The screen is on, but what it shows is wrong.”
Start at Computer monitor display issues
“The second screen is not there at all.”
That is Second monitor not detected
“The touch has stopped answering.”
Go to How to fix phone screen unresponsive touch problems
“Everything stopped and the screen went blue.”
That is not the panel: Blue screen error
“I only want a picture of what is on the screen.”
The other half of this subject is in Software download

Which half is broken

Before anything else, and before any money. A fault in the panel and a fault in the machine behind it look the same from the outside and are not in the same price range. These guides separate them first.

Half black screenThree built-in tests, one per device, and the one that runs with the computer unplugged.Open this guide → A display backlight that stops workingThe one fault where the built-in test cannot help, and the four outputs you can still read.Open this guide → Computer monitor display issuesWhat each test proves — and a splitter can never extend a screen.Open this guide → Second monitor not detectedWhen the screen is not seen at all, which is a different failure from one that shows the wrong thing.Open this guide → Dead pixel repair on a laptopA dead pixel and a stuck pixel are not the same fault, and only one of the two has anything worth trying.Open this guide →

Marks on the glass

One shape of mark at a time, because the shape is the diagnosis. A straight line, a spreading patch, a bruise under pressure and a half-white screen have four different causes and four different answers about repairing them.

A vertical black line on a laptop screenThe commonest mark of them all, and the one where the check before the quote matters most.Open this guide → White horizontal lines on a phone screenWhat the lines mean on a phone, where almost nothing inside can be replaced on its own.Open this guide → A blue spot on a phone screenTwo different faults asked in the same words, and the screenshot that separates them in a second.Open this guide → White spots on a phone screenWhere they come from, whether they spread, and the honest answer about the ones that are sold as fixes.Open this guide → Black marks on a computer screenInk-like marks that are not ink, and the one test that separates them from something on the surface.Open this guide → Black pressure marks on a laptop screenThe bruise a closed lid leaves, what it does next, and what it costs to be rid of.Open this guide → Half the laptop screen gone whiteA whole half lost at once points somewhere specific, and it is not the glass.Open this guide →

A picture that will not settle

Not marked and not blank: moving when it should be still, or still when it should answer. These two are the faults most often blamed on the panel and least often caused by it.

How to fix a laptop green screenWhich colour frame of the built-in test gives a cast away, and why it is never the green one.Open this guide → A monitor blue colour problemThe test that runs with the computer unplugged, and the setting that moves the picture the wrong way.Open this guide → Monitor flickeringThe diagnosis for the monitor itself, not the settings walk-through for one operating system.Open this guide → A screen that ignores youFour steps from Apple, and one of them ends by telling you to change your charger.Open this guide →

Not covered here. It will not send you to a shop before the check that says whether a shop is needed. That check is the first thing on every floor here, it costs a minute, and on a fair share of these faults it ends the matter without anybody being paid.

It will not repeat the pressing, rubbing and tapping routines that circulate for spots and stuck pixels as though they were free. Where a guide describes one, it says what the technique can do, what it cannot, and what pressing on a damaged panel risks making worse.

It will not pretend a phone and a laptop are the same machine. What can be replaced on its own, what comes only as an assembly, and what a repair is worth are different questions on the two, so the guides keep them apart instead of writing one answer that fits neither.

And it will not carry affiliate links, which in this subject is not a small promise: replacement panels, repair bookings and screen-repair kits are the entire economy around these questions, and they pay best to whoever reaches the reader while the reader is worried. That is the position every page here is written from the other side of. What holds instead is simple: the first question is always which half of the machine is broken, and answering it costs a minute rather than a quote.

Written by Alberto Gulotta

Founder and editor of AI Tools Primer, writing from Palermo, Italy. Thirty-five years of taking computers apart, starting with a Commodore 64 — the long version is on the about page.

Something wrong on this page? Write to aitoolsprimer@gmail.com and it gets fixed.

Written on 11 September 2026 · last checked 16 September 2026.

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