See if Apple will replace the board, do not mention that you changed the back light board. I know the LEDs do not show this but experience says this is the problem: https://support.apple.com/en-us/HT203787
Due to high demand Apple has extended this period of time in a lot of cases, mention the term, "Warranty Exception".
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
If the iMac falls within one of the above the serial number categories and is confirmed to have the graphics card issues, Apple will replace the AMD card free of charge up to three years from the computer’s purchase date. Additionally, if an affected iMac user had their graphics card already replaced at cost, the customer is said to be eligible for a refund.
http://9to5mac.com/2013/08/16/apple-opens-graphics-card-replacement-program-for-some-mid-2011-imacs/