In DFU mode, the screen remains completely black . If you see an Apple logo or "Connect to iTunes" icon, you are in Recovery Mode, not DFU.
This is the most common way to "flash" your firmware. It uses your computer to download the latest signed version of iOS from Apple's servers and install it. iphone firmware reset
Before performing an iPhone firmware reset, make sure to: In DFU mode, the screen remains completely black
You do not need a firmware reset for everyday issues like a forgotten passcode. However, you should consider this nuclear option if you experience: In DFU mode