Respuesta a "How to fix "vertical lines" on my iMac 27 late 2009"24" 2009 iMac... EUREKA!!!! This is a long time since the first post but I found your comments really helpful... thank you. I previously had the HD renewed, a new fan put in etc. put vertical lines persisted. I ended up buying a new one - from Apple - which I realise now I didn't need to. After months of fiddling, there is NOTHING wrong with my original Mac. I stripped everything off it and restored to factory settings. I then rebooted (CTRL OPT P R) and (after an overnight reload) hey presto! I'm back to Sierra OS with no lines/distortion. It's been running for 6 hours now and no problems. It is blatantly an Apple intentional problem or 'fix' just to get us all to ditch our perfectly good Apple products so they can make a bit more money off the faithful! Disgraceful.