I have an issue with my iMac 2008 (8.1) (vertical lines).

And I don't know where is the problem GPU/LCD.

Someone said I must buy mini DVI to test if the the screen is defective using a external screen?

How do I test if my LCD screen or GPU is defective.

  • This Mac does not have a Thunderbolt port, but a Mini DVI one for video output. VGA or HDMI will be ok for testing. Please edit your question because as it stands it's difficult to understand your problem. Thanks! Commented Jul 26, 2015 at 17:09
  • That depends on what choices you have as second monitor (or TV) to test. Based on that you will need the correct adapter (mini dvi to ??).
    – Ruskes
    Commented Jul 26, 2015 at 18:30
  • @Quatto Please remove the acception of my answer, so I may delete it.
    – klanomath
    Commented Jul 26, 2015 at 18:38
  • Does the iMac present any other symptoms? When I had a similar issue I found my iMac in a constant reboot cycle, though the initial boot up sequence did not have vertical lines.
    – forquare
    Commented Jul 27, 2015 at 16:22

1 Answer 1


The other commenters are correct. With an iMac you need to try an external monitor to determine if it is the screen or the GPU.

An Apple Genius appointment would cost you nothing and they could test it for you if you don't have access to another monitor and/or the right cables.

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