Do you see the white mark when running the LCD BIST (Built In Self Test) ?
* Turn the Dell laptop off
* Connect the AC adapter (charger) to the laptop
* Disconnect all peripherals (mouse, keyboard, monitor, USB key, etc.) from the laptop
* Press and hold the D key and then press the power button. The LCD BIST mode should appear
* Release the power button but continue to hold the D key, until you see color bars on the LCD. The LCD will display multiple color bars and change colors on the entire screen to red, green and blue
* Press the Esc key to Exit