I am troubleshooting a Thinkpad T400 notebook. It uses an LED-backlit screen. The original screen was dim. I have replaced the LCD cable, "inverter" (not a true inverter but it does connect the LED power), as well as the LCD itself. The screen is still dim.
The only hunch I have right now is that if it's possible the LCD dimming switch that activates when the screen is closed is stuck, but I can't identify where this switch is in the IBM/Lenovo teardown and guide for this model.