Internal Display not working, External does.

Hi, I have read a lot about this issue and have done some of the things to rectify this issue but could not resolve this.

First, When I power on the Imac, I can hear chime, Hard drive and fan noise, Three led are lit on the logic board, while fourth remains off. I checked by changing Graphics Card, Inverter Board, Power Supply and lvds connector, and even LED. But still not able to see anything on internal LED while when I connect External monitor, I can use it as a normal mac.

Please help, What can be the issue.