An iMac screen fault is harder to read than a MacBook's, because a line, a flicker or a dim patch could be coming from the panel, the cable behind it or the graphics chip, and there is no way to unplug the screen and test each part on its own. Each of those causes calls for a completely different repair at a completely different price. The checks below, roughly in the order we run them at the counter, will usually tell you which camp your iMac falls into before you book anything.
Lines at boot versus in macOS
Watch the grey startup screen, with the Apple logo and progress bar, before macOS has finished loading. If the lines are already visible there, nothing software-related has started running yet, so a driver update, an NVRAM reset or a fresh macOS install will not touch them; the cause has to be physical. This matters because a lot of generic troubleshooting advice assumes any screen glitch is a graphics driver problem you can update your way out of. On an iMac, note carefully whether the fault looks identical at boot and on the desktop, only appears once macOS has loaded, or only shows up during heavier graphics work like video playback or gaming, since that timing is one of the most useful details we ask for when you bring it in.
External display test
Any iMac with a Thunderbolt port, which covers essentially every 27-inch model since 2011 and most 21.5-inch models since, can drive an external monitor from that same port; on 2020 and later 27-inch models an HDMI output works the same way. Connect a monitor and mirror or extend the desktop. A clean, stable picture on the external screen while the built-in display still shows the fault means the graphics chip is producing a clean picture and the fault lives in the iMac's own display path, either the panel or the cable and connector feeding it. If the external monitor shows the same lines or artefacts, the fault is upstream in the graphics processor, and no amount of screen or cable work will fix it. No other check you can do at home tells you as much in five minutes.
Panel faults
A faulty panel produces lines, blotches or a dim region that stay fixed in the same place on screen: present at boot, in macOS, in Safe Mode, and unaffected by what is actually being displayed. On the 27-inch 5K iMac the panel is a large laminated assembly held in place with adhesive strips rather than screws or clips, and the same adhesive-strip mounting carries through the 21.5-inch 4K and non-Retina models from 2012 to 2019. The earliest 2009 to 2011 models instead hold the glass on with magnets, with the LCD itself screwed in underneath. Either way, reaching the panel means releasing that adhesive or those magnets cleanly and reseating a new panel without trapping dust behind the glass, which is bench work rather than something done at the counter in a few minutes.
Cable and connector faults
The panel connects to the logic board through an LVDS or eDP cable running a short distance inside the chassis, and like any cable it can fray, work loose at its connector, or develop a poor joint after years of the iMac heating and cooling each time it is used. The giveaway is that a cable fault changes rather than stays constant: lines that shift, flicker or clear when you gently tap the top or side of the case, or a picture that looks fine when the iMac is cold and only starts showing a fault once it has been running for twenty minutes and the chassis has warmed through. A connector reseat or a cable replacement is a much smaller job than a full panel swap, and it is worth testing for exactly that pattern before assuming the panel itself has failed.
GPU on 2011–2013 iMacs
The 27-inch iMac from 2011 to 2013 shipped with a discrete AMD or NVIDIA graphics chip mounted on the logic board, and this generation has a recognised pattern of that chip failing as the machines age. Rather than clean, straight lines, a dying GPU on these models tends to show up as coloured artefacts, tearing or garbled blocks across the image, often together with freezes or an unexpected restart, and it frequently appears or worsens specifically after the iMac has been running a while and the board has warmed up, pointing at a stressed solder joint rather than the screen or its cable. This is a logic board repair, not a display job, and because the fault originates on the board, the external display test above will show the same artefacts on a second monitor too.
Is it worth fixing?
The honest answer depends entirely on which of the three faults you actually have. A cable or connector repair is the cheapest outcome and worth doing on almost any iMac still in regular use. A panel replacement is the bigger job, and we only quote it once the bench has confirmed the screen itself is at fault rather than the cable behind it, since swapping a panel to chase what turns out to be a loose connector wastes your money. A logic board repair for the 2011–2013 GPU fault starts from RM500 with a one to three day turnaround and a three to six month warranty, and is usually worthwhile on a 27-inch iMac that is otherwise in good shape, since the rest of the machine still has plenty of life left in it. Where an iMac is old enough that a repair would cost close to what a comparable secondhand machine sells for, we say so plainly at the counter rather than quote a job we do not think makes sense.
When to bring it in
Bring the iMac in on its own; you do not need the keyboard, mouse or original box. We run the firmware-boot check, the external display test and the tap test on the bench, in front of you, and tell you plainly whether it is the panel, the cable or the graphics chip before quoting anything, and the diagnosis costs nothing. See the iMac repair page for what a screen or board job typically involves, or if the bench confirms a graphics fault rather than the display itself, logic board repair covers the component-level process and warranty in more detail. The price guide lists every repair we quote, screens and boards included.