XPS 13 Plus 9340, backlit keyboard and function keys (Fn)

Remove the base cover and disconnect the battery from the mainboard.  Ensure that the cable to the keyboard that powers the backlight is connected properly,  Hold the power button for 30 sec with the battery disconnected.

Reassemble and check again.

Service manual:

https://www.dell.com/support/manuals/en-us/xps-13-9340-laptop/xps-13-9340_owners_manual/working-inside-your-computer?guid=guid-dd3ae169-a824-4f1d-832e-585b176f6faf&lang=en-us

@ejn63​ 

Thanks for the swift feedback.
I just went through your suggestions: disconnected the battery and held the power key for 30 sec. The laptop took longer to boot and, after a while, returned to the same BIOS window, where the option to choose the backlit feature was missing.

Any other suggestions?

@XPS_R​ Attempt to load the BIOS defaults next:

1. In the BIOS click on Load Defaults
2. Select BIOS Defaults
3. The unit should automatically restart. Enter the BIOS again
4. Check to see if the backlight options return within the BIOS

 @DELL-Daniel V​ 
I tried these steps, but it's the same.

@XPS_R​ If the BIOS is at its latest version, hard resets were attempted, and loading the BIOS defaults still do not return the backlight function, then unfortunately it would seem to be a hardware failure. With the backlight options missing in the BIOS, thats letting you know the board is not detecting that functionality. This being the capacitive touch version, then replacing the keyboard will not be the fix here. The function row LEDs are built into the palmrest assembly so that would need to be replaced should you want to look into a repair. 

If already out of warranty, click here for the Dell out of warranty offerings. The palmrest part you should need is YNTWD. 

@DELL-Daniel V​ 
Thans

Hi,
I've an update. The backlit keyboard is back. However, the Fn keys/the LED row is still off.
Is it a hardware or a software issue?

Run the keyboard test (Dell diagnostics - F12 at powerup).  Do the keys illuminate during the test?

@ejn63​ 

Yes, they did.

Now everything has returned to its normal state.

Thank you so much for your guidance.

Fn key behavior on the XPS 13 Plus can be inconsistent until you lock in the right BIOS setting. Check whether Fn Lock is enabled in BIOS because on some 9340 configurations the default toggles between function key priority and media key priority in a way that is not obvious from the keyboard behavior alone. Also worth checking whether the keyboard backlight setting is persisting after restart because there is a known behavior where the backlight resets to a default level on cold boot regardless of what you had it set to previously. Did the issue start from day one or after a specific BIOS or driver update? 

@DELL-Daniel V​ I have a similar problem with the Latitude e7470: after replacing the keyboard with the same one as the Swedish layout, the keyboard highlighting stopped working and the Keyboard Illuination section disappeared from the BIOS and the ability to turn on/off the highlight. The flashing on the old keyboard with a German layout was working. Resetting the BIOS to factory settings did not work. Can you suggest some possibilities for solving the problem?