[LaLamoto]

I just updated my phone yesterday. Now my back light isn't working. Went to settings under device clicked display & wallpaper then touch key light duration. No progress. I also tried resetting and powering on then removing battery. In addition I also removed new downloads. Any advice?

[nahoku]

Did you update from KK to LP? In any case, it's recommended that you wipe cache whenever you update so any KK junk won't interfere with LP. I'm on Verizon myself and the following method works for my phone although it's a T-mobile link. If the method doesn't match Sprint's, you can always search on Google so you do the right steps...
https://support.t-mobile.com/docs/DOC-14686

Note... wiping cache doesn't mess with your apps or files, but you never know... always good to have a back up just in case.

[LaLamoto]

Thank you for your advice. My phone is currently updated to Lollipop. After more than an hour of methods I finally realized the issue was power saving mode!!!
Here's a few tips just incase you encounter this issue.. Go to Settings, click Power Saving, then click Restrict Performance. Be sure to uncheck Turn Off Touch Key Light.

[nahoku]

I'm glad you got it sorted. Yes, Power savings can disable you capacitive buttons. I thought you only had a problem with one of the buttons (back button), so I thought KK might be glitching your settings.