Many of you are facing the error (1) or error (6) while downgrading the firmware. In this situation, you need to put your iPhone in DFU mode, than restore or downgrade the firmware.

METHOD 1: 10/10 Method

Step 1. Turn on the iPhone. Turn on iTunes. Connect iPhone to Computer thru USB cable.

Step 2. Hold Sleep and Home button together for exactly 10 seconds (use your computer's built in clock, turn on the seconds counter). You have to be exact.

Step 3. After the 10 seconds above, release the Sleep button and keep holding down the Home button for another 10 seconds. Release after the 10 seconds.

Step 4. iTunes should now tell you that it has detected an iPhone in recovery mode. Click ok and your set to restore your iPhone.

(ZZ: note, if you're on iTunes 7.6 or higher, you need to be connected to the internet before iTunes will allow you to restore your iPhone. For iTunes 7.5 or below, no need to be connected)
Video
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METHOD 2: ZiPhone DFU (for Firmware 114 and below only)

Step 1. Launch iTunes to enable your iPhone to pair with iTunes. Close iTunes after it detects your iPhone (at that point its paired already)

Step2. Launch ZiPhone. Click "Mode", Click "Restart in DFU Mode"


The other linkS:

 Aside from how to put it into DFU and ecovery mode, the article below teaches you how to get out of those 2 modes.

How to Get your iPhone (in/out of) Recovery Mode and DFU Mode
via ilikemyiphone

Both recovery and DFU modes are often confusing which just adds to the frustration when accidentally put into either of these modes. You don't have to restore completely in iTunes. That takes up a lot of time (around 20 minutes and if you have data, it may be longer to get it all back up and running). You can give the following combinations a shot and I have tested them both on iPhone 3.0.

Steps to recover from DFU mode :

* Connect your iPhone to your computer.
* Hold down the home button and the power button for about 10 seconds.
* If your phone is connected to your computer you'll hear a beep as when a USB device is disconnected.
* Next just press the power button and your phone will startup as usual.

If you've accidentally put your iPhone in DFU mode, the steps mentioned above should do the trick.

If you're in DFU and mid-way through a failed restore, then you will need a complete iTunes restore to get you out of the DFU mode.

Steps to recover from Recovery mode :

* Hold Power Button + Home button for 15 Secs.
* Release buttons.
* Hold Home button for 2 secs.

DFU (Device Firmware Upgrade) mode is not the same as restore mode. DFU mode bypasses the current OS installed and allows you to upgrade or downgrade your OS. If you are getting errors trying to restore in iTunes, DFU mode is very likely for you.

DFU mode is not recovery mode. Recovery mode has a picture that says connect to iTunes. In DFU mode, the iPhone screen backlight is not even on and the display is blank. If you have anything on your display you are not in DFU mode.

This is how to achieve DFU mode. You can do it on every phone:

Method 1)

* Attach the phone to the pc
* Turn the phone off
* Hold power and home together for *exactly* 10 seconds
* Release power but keep holding home until the pc beeps as a USB device is recognized.
* At no point will the display come on. Now your restore should work.

Having problems? Try adjusting the 10 seconds to be slightly less like 9 seconds, then 8, then 7. It's a tricky timing but you will eventually get it.

Method 2)

* Hook up the phone powered on to the PC
* Now hold home / power until the phone turns off and keep holding for 10 seconds. If the phone ever turns on during this phase,start over and hold for less time.
* Now release power and continue holding home until the PC recognizes the phone.
* I don't like this method as much because there are "rumors" that it may cause some 1604 errors in the restore. I am not convinced of that though.

Method 3)

* Attach the phone to the pc
* Turn the phone off
* Press and hold power. Keep holding power. As soon as you see display on the screen of any sort press and hold home.
* Hold power and home together for *exactly* 10 seconds
* Release power but keep holding home until the pc beeps as a USB device is recognized.
* At no point will the display come on. Now your restore should work.

Having problems? Try adjusting the 10 seconds to be slightly less like 9 seconds, then 8, then 7. It's a tricky timing but you will eventually get it.

Have MAC OSX 10.5.6 or a MAC Book slim? You have a problem entering DFU mode. Here is an easy fix from the dev-team blog:

1. You will need an account with ADC (Apple Developer Connection) this is free and takes a few minutes to sign up, you should read the terms and conditions carefully and you should only sign up if you are thinking of developing applications in the future - apple.com
2. Download the disk image "IOUSBFamily Log release for Mac OS X 10.5.5 Build 9F33" (yes, that is a "5" in 10.5.5 - this is a developer debug package of the USB kernel extension).
3. Install IOUSBFamily-315.4.1.pkg from within the disk image
4. Reboot your system!