Why does my phone's brightness keep fluxuating?

[isaacjh02]

After a couple minutes with me using my phone, there is no change and my phone's brightness goes very low and slowly get brighter again. I don't have auto brightness enabled and i have no idea why this is happening. If it matters I did a factory reset and my Android version is 6.0.1.

 

[B. Diddy]

Welcome to Android Central! Try booting into Safe Mode, which temporarily disables all 3rd party apps. https://support.t-mobile.com/docs/DOC-21007

If the problem disappears in Safe Mode, then something you installed is causing the problem. You may have to uninstall apps one by one until the problem disappears. If the problem persists in Safe Mode, then it's more likely to be a hardware or firmware problem.