How do I troubleshoot white screen in dell inspiron 6400?

Anzar, if your Laptop screen is white and an external monitor does not give you a display either, sounds like a bad GPU. The white screen is nothing but a backlight that is on. So your inverter sounds to be okay as well. Try to reset the CMOS for your system. Remove the AC power, the RAM, and the battery. Hold the power button for about 30 seconds. This should discharge the components. After that, reinstall RAM, battery and power up to see what you get. If that does not work, time to disassemble your laptop and take a look at the video controller. It is either ATI Mobility Radeon X1300 or X1400 or nVidia GeForce Go 7300, and may need a reflow. Hope this helps, good luck.