The most likely cause of the issue is a defective inverter. An inverter is a device that converts the direct current (DC) power from the laptop battery or adapter to the alternating current (AC) power that the LCD backlight bulb needs. The backlight bulb is a fluorescent lamp that illuminates the LCD screen and makes the image visible. If the inverter is defective, it may not provide enough power to the backlight bulb, or it may stop working intermittently. This can result in a dim, dark, or blank screen, even though the LCD panel is still functioning and displaying a faint image.

To troubleshoot a defective inverter, the technician can follow these steps:

Connect an external monitor to the laptop and check if the image is displayed normally. If the external monitor works fine, it means that the laptop video card and driver are not the cause of the issue.

Shine a flashlight into the LCD screen and look for a faint image. If the image is visible, it means that the LCD panel is not the cause of the issue.

Check the inverter cable and connector for any damage, loose connection, or corrosion. The inverter cable is a thin wire that runs from the laptop motherboard to the inverter board, which is usually located at the bottom of the LCD screen. The inverter connector is a small plug that connects the inverter cable to the inverter board. The technician can try to reseat the inverter cable and connector, or replace them if they are faulty.

Check the inverter board for any damage, burn marks, or blown fuses. The inverter board is a small circuit board that contains the inverter and other components. The technician can try to test the inverter board with a multimeter, or replace it if it is defective.