When an iPhone is in Device Firmware Upgrade mode, it is unresponsive and the screen is black. The phone doesn't respond to input from the Home or Sleep/Wake button. While in DFU mode, the iPhone seems as though it is not powered on at all. There is a simple way to exit from DFU mode: Reset the iPhone.
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How to Exit DFU Mode
As long as there is no underlying physical hardware issue or unknown software corruption, an iPhone can be taken out of DFU mode.
Press and hold the Sleep/Wake button and the Home button simultaneously for five to 10 seconds or until the Apple logo appears. Do not release the buttons until you see the Apple logo.
After the Apple logo appears, release the Sleep/Wake and Home buttons. The iPhone is in the process of restarting and will return to the Home screen.
What Is DFU Mode?
As the name implies, Device Firmware Upgrade mode is a state that the iPhone can enter to allow a firmware upgrade. DFU Mode prevents all extraneous hardware and software components from being used, allowing only the firmware of the device to operate. On an iPhone, DFU mode allows the iPhone's operating system iOS to be updated or modified. The only way to tell if the device is truly in DFU mode is to connect it to a computer and see if the computer recognizes it as an attached device.
When to Use DFU
There are few reasons, if any, for a typical user to put an iPhone into DFU mode. DFU mode is used by Apple Store Geniuses and other Apple-certified technicians to restore the iPhone's iOS when other troubleshooting methods fail. It is also used by expert users to modify the iOS to install alternative, non-supported features on the iPhone; this process is referred to as jailbreaking, and it may void the product warranty.