DFU means Device Firmware Update. If a restore using Recovery Mode doesn't work you will want to use DFU Mode as a last resort. When placing your iPhone into DFU Mode it does not load the OS before attempting the restore.
To put the iPhone into DFU mode so you can do an iTunes firmware restore follow these steps:
METHOD ONE
Step One
Open iTunes and connect the iPhone to your computer.
Step Two
Press and hold the Home button and the Sleep/Wake button at the same time.
Step Three
After exactly 10 seconds release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
The iPhone will look like this:
METHOD TWO
Step One
Plug in your iPhone and turn it off.
Plug in your iPhone and turn it off.
Step Two
Hold down the "Home" and "Sleep/Wake" buttons for about twelve seconds. You'll briefly see the silver Apple logo and then your phone will appear to turn back off.
Hold down the "Home" and "Sleep/Wake" buttons for about twelve seconds. You'll briefly see the silver Apple logo and then your phone will appear to turn back off.
Step Three
Release the "Sleep/Wake" button but continue to hold the "Home" button until you see a message in iTunes stating that it has detected an iPhone in recovery mode.
Release the "Sleep/Wake" button but continue to hold the "Home" button until you see a message in iTunes stating that it has detected an iPhone in recovery mode.
