Symptom 1: random horizontal or vertical lines
If you see thin coloured lines that come and go — usually vertical on Pros, horizontal on Airs — that's a flex cable issue. The display cable runs through the hinge and gets fatigued over thousands of open-close cycles. On the 2016–2019 MacBook Pro the 'flexgate' problem is so well-known Apple has an extended service programme for it.
Try opening the Mac slowly and watching when the lines appear. If they're worst at 30–60 degrees of hinge and clear up when fully open, it's the flex cable. The fix is a new cable or a full display assembly — at AppleForce we keep both in stock for the affected A-numbers.
Symptom 2: full-screen flicker every few seconds
If the entire screen blinks dark for a fraction of a second every few seconds, this is usually a GPU issue. On Intel MacBooks with discrete AMD graphics (especially 15-inch 2018–2019 A1990), the dGPU can develop solder fatigue from heat cycling. The blink happens when the OS switches between integrated and discrete GPUs.
Workaround: install gfxCardStatus and force the iGPU only. This reduces performance but stops the flicker. Long-term fix is a reflow or GPU package replacement, which we do daily on Pro 2018–2019 boards.
Symptom 3: screen tints pink/green or has dead patches
If the panel itself looks off-colour or has zones that don't change, the LCD is damaged. This often happens after a small impact you may not have registered as a 'drop' — closing the lid with an earbud trapped under it, sitting on the closed Mac, even temperature shock from AC to humid air.
Connect an external monitor via USB-C or HDMI. If the external looks perfect, your internal panel is the only damaged part. A full Retina display assembly swap fixes this; we always use genuine Apple panels with True Tone preserved.
Software causes worth ruling out first
Sometimes flicker is software. Try: System Settings → Displays → uncheck 'Automatically adjust brightness'. Boot in Safe Mode (hold Shift on Intel, hold power on Apple Silicon then choose the volume and click Continue in Safe Mode). If flicker stops in Safe Mode, a third-party app or extension is the cause — usually display-mirroring apps, screen recorders, or old DisplayLink drivers.