Apple has determined that some AMD Radeon HD 6970M video cards used in 27-inch iMac computers with 3.1GHz quad-core Intel Core i5 or 3.4GHz quad-core Intel Core i7 processors may fail, causing the computer’s display to appear distorted, white or blue with vertical lines, or to turn black. iMac computers with affected video cards were sold between May 2011 and October 2012.
The last four characters of the serial number must contain one of the following groups (for example, xxxxxxxxDHJQ):
● DHJQ, DHJW, DL8Q, DNGH, DNJ9, or DMW8
● DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, F610
You can see the graphics issue demonstrated in the video below. If you have one of the specified Macs, Apple will replace the AMD graphics card free of charge for three years from the computer's purchase date. If you've already replaced the card at your own expense you may be eligible for a refund.