Pink Vertical Lines in iMac 27″ 2009 and 2011 is common issue. We can troubleshoot this issue by following some easy and simple tricks. First we need to find the exact issue if it is screen problem or GPU (Graphics) problem. The trick to prove if the display or the GPU is bad is to plug in an external monitor. If the external display looks OK, the internal display is bad. If the external display looks bad then it’s a GPU failure. We still have a few of these (EMC 2374) which we use as test stations both are running newer versions of OS-X Yosemite & El Capitan without any problems. Over the years many had GPU failures when we pushed them hard. Are you running some heavy graphics on your system like gaming? That will kill your system as the current near real life graphics is just to much for this old boy.
Graphics issue can be solved by using simple trick.
Disassemble your iMac and remove Graphic Card.
Apply normal heat with heat gun.
Clean old paste and apply new paste.
Don’t apply too much heat normal heat is enough 180 to 200 degrees.
insert Graphic card into iMac and turn it on.
if problem not solved then you need to change your graphic card with new one.
But 90 percent cases solved by using this simple method.
I solved many iMac GPU problems by using this method.
For more detailed info just leave comment below we will reply ASAP.