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Hi my late 2012 iMAC 27 as it boots up I get the screen covered in multi coloured bars and after boot up the screen flickers I is 100% ok when running on an external monitor I this a failed graphics card or the screen itself PS where is the graphics card I have the screen panel off and can’t see anything obvious Thanks
Sadly this sounds like you have a bad display assembly. This can happen when the wrong tools are used to remove the display. Here’s the proper tool you need: iMac Opening Wheel and i recommend getting two sets of these Plastic Cards. As you’ll need new adhesive strips you might as well get the kit [linked product missing or disabled: IF174-005-1] which comes with the cutter wheel. And lastly here’s the needed part: 27” iMac Display Assembly (used) Apple P/N 661-7169 The new ones do seem to be in short supply. Here’s the guide to put it in iMac Intel 27" EMC 2546 Display Replacement
