Inspiron 3162 Backlight issue

 backlight is off from booting itself

During diagnosis (power key+D)

I'm using dell 3162 , suddenly it's Backlight stopped working. But it's lcd is working fine. Some times the Backlight flickers.But ,while troubleshooting display using power button + D ,the Backlight turns on and turns off when then diagnosis is finished.

I tried reinstalling windows, reinstalling video driver,installing Linux, checking for loose connections in the mother board. Still I have no clue ..

How to fix it?