In our 4 Rs of iPhone troubleshooting, we note that restoring the iPhone as a new phone can resolve various issues. However, some problems may require that the phone is restored while it is in DFU (device firmware upgrade) mode. Doing so may resolve issues with Exchange, an inability to send and receive MMS messages, device slowness and other problems.
To perform a DFU restore, follow these steps:
1. Backup your phone and preserve data. Follow the steps in this Apple Knowledge Base article to transfer your purchases and backup your iPhone.
2. Put iPhone into DFU mode. Connect your iPhone to your computer and open iTunes (if it doesn’t open automatically). Hold down both the top (sleep/wake) and home buttons for exactly 10 seconds, then release the top (sleep/wake) button but keep holding the home button until iTunes displays a message stating that a phone in recovery mode has been discovered.
3. Restore. Press the restore button and allow the phone to complete the restoration data. After it is done, select setup as a new phone.
4. Restore your backup (optional). Next, you can restore the phone again in the normal fashion (not DFU mode) using the most recent backup rather than setting up as a new phone. However, this may cause the original problem to return in some cases. If the problem returns, follow steps 1-3 again and do not restore your backup.