iMac screen flickering or showing lines? What it means
5 November 2026 · Mac guides
Lines across the picture, a screen that flickers, or one that goes dark for a moment and comes back: display faults on an iMac are frustrating because the screen is the whole computer. The good news is that a simple test tells you a lot about where the fault is. Here's what the symptoms usually mean and what you can check before booking a repair.
The screenshot test
This is the single most useful check, and it takes a few seconds. When the lines or flicker are showing, take a screenshot by pressing Shift-Command-3. Then open the screenshot from your desktop on another device, such as your phone or a different computer.
- The lines or glitches are not in the screenshot: the iMac is producing a clean picture, and the problem happens after that, in the display panel or the cable that connects it. This is a screen fault.
- The lines or glitches are in the screenshot: the picture is already wrong before it reaches the screen. That points to the graphics chip or the logic board, or occasionally to software.
If you have a spare monitor and your iMac has a port that supports an external display, connecting it gives a similar answer: a clean picture on the external screen means the fault is in the iMac's own display.
What the symptoms usually mean
Vertical or horizontal lines
Thin coloured lines that stay in the same place, and don't appear in a screenshot, are usually a fault in the panel itself or its connection. They often appear after the iMac has been moved or knocked, and they rarely go away on their own.
Flickering or brightness changes
Flicker can come from the panel, the backlight, the display cable or, less often, from software. Try lowering and raising the brightness, turning off automatic brightness in System Settings, and starting in safe mode. If it still flickers in safe mode and the flicker doesn't show in a screen recording, it's almost certainly hardware.
The screen goes black but the iMac keeps running
If you can still hear sounds or see a faint image with a torch, the backlight may be cutting out. On older Intel iMacs this is often a power supply or backlight board fault rather than the panel.
Distorted picture, artefacts or crashes
Blocky patterns, scrambled colours or the iMac freezing when the picture goes wrong, especially if they show up in screenshots, suggest a graphics fault on the logic board. On older iMacs with separate graphics chips this is a known type of failure. These can sometimes be repaired at board level. See logic board repair.
Quick checks before you book a repair
- Restart the iMac. A one-off glitch that clears after a restart and never returns may have been software.
- Install macOS updates if your iMac still receives them. Display driver bugs do happen, and they're fixed in updates.
- Start in safe mode and see whether the fault appears. Apple's support site explains how for Intel and Apple silicon iMacs.
- Note when it happens: from cold, after an hour of use, only on certain apps, or only after the iMac was moved. That helps a repairer find the fault faster.
Is it worth repairing?
An iMac screen is a large, bonded part, so a full panel replacement costs more than a laptop screen. Whether it's worth it depends on the model:
- Recent 24-inch Apple silicon iMacs are usually worth repairing, especially if the rest of the machine is in good condition.
- 27-inch Intel iMacs have excellent 5K screens and many are still very capable. A cable or backlight repair can be good value, and a panel replacement can still make sense if the iMac is otherwise healthy.
- Older iMacs that no longer receive macOS updates are worth weighing up carefully. Sometimes the best option is to use the iMac with an external monitor or to recover its files and replace it. Our too expensive to repair? page covers those choices.
Back up while it still works
A flickering screen often gets worse over time. If you can still use the iMac, back it up now with Time Machine. If the screen fails completely before you do, a technician can usually still reach your files, but it's easier while you can see what you're doing. If the drive itself is failing, see Mac data recovery.
iMac screen repair in London
We diagnose iMac display faults by testing the panel, cable, backlight and graphics separately, so you know exactly what's wrong before any work starts. See iMac repair and Mac screen repair for more details.
Get a quote
Call 020 7394 2525 or send us an enquiry. Bring your iMac to 80 Willow Walk, London SE1 5SY, or use our bike collection in London or courier collection UK-wide. We are open 9am to 6pm, Monday to Saturday.