I need to know how to replace the backlight

Hi @combatman ,

I assume that you have proved that the backlighting is faulty by either using an external monitor or shining a torch at an angle close to the screen to see if the images are still there.

If this is the case, then the backlighting LEDs may not be faulty, it may rather be either a faulty lid switch, a faulty LCD cable (or cable connections at either end) that supplies the power to the backlighting in the LCD screen or it could be a faulty screen as well, (the lights are integrated into the screen).

The lid switch (not mentioned in the manual) is usually a magnetically operated switch, with the magnet in the lid and the switch in the body, that either disconnects the backlight power when operated OR that sends a signal to the BIOS to switch off the backlight power by software control.

You may have to open the laptop to find the problem

Here is a link to the service manual for a Lenovo Z400/P500/Z500. I cannot find a manual for a P400- this manual is from the Lenovo website for a P400 but does not mention it in the manual. As best as I can find out the only difference between a P400 (20211) and a Z400 is the screen resolution, 1600 x 900 vs. 1366 x 768 but this may be wrong. See image for location of service manual download

(click on image to enlarge for better viewing)

Hopefully this is of some help.