Last week I had a new motherboard installed on my Asus Q500A laptop. When I got it back the keyboard backlights no longer worked. This wasn't a problem before I sent it out for repair.
This issue is caused by the BIOS being changed. Here is a post about it in the ASUS forum:
This fixes the issue in this one case : '7C' replaced by 'FF' in the EEPROM
make a usb boot drive, set your bios so flash drive comes first at start up, put bt.exe and fixmykb.bat to the usb key, run fixmykb.bat
Asus computers with a backlit keyboard come with the flag '7C' written in the cnfg page of the EEPROM.
It's a small sticky memory on the motherboard. Apart from the BIOS.
The flag address is cnfg DD (and not 27 as written in many forums)
It seems, again reading forums from all over the world, that not all Asus RMA centers know that !
Some replaced, the keyboard, the mother board (!), send the computer back without fixing it, but in most cases, fixed it without any explanation.
For months now, ASUS does not release a patch. A very simple app from windows could write this back to EEPROM. But here's the Asus problem : one flag in this eeprom (27) tells windows installer what version (like pro or ultimate edition) of windows to install... So may be some guys over there just dont want the BT.exe utility to be spread around.
In most cases, if you flashed the latest BIOS available, the latest ATK drivers and have your back lite keyboard not working, this fix will give the light back to you !
At last, some few users had the keyboard power ribbon unplugged. Before doing the hardware work to pull it out and risk damaging this fragile plastic stuff, try this fix.
WORKS ON G73 and U80V (just beginning the list). Brings also the light back to U80V illuminated trackpad.
ASUS Q500a 15.6"...
on Aug 16, 2015