iMac backlight only on lowest setting (LCD panel or Inverter defect?

hey guys, I have an iMac 2011 27 inch. The problem with this iMac is that the backlight only works on the lowest setting. If I increase the brightness the screen will go black, but the desktop is working because I can see it using a flashlight. The icons and desktop is visible with a flashlight.

GPU is probably working. Using an external monitor does not have this issue.

Diagnostic LEDs 1,2,3,4 are on.

Do you guys think it is the inverter or the actual LCD backlight itself ? thanks