Error 1600

Error 1604

This error is often related to USB timing. Try changing USB ports, using a different dock connector to USB cable, and other available USB troubleshooting steps (troubleshooting USB connections. If you are using a dock, bypass it and connect directly to the white Apple USB dock connector cable. If the issue persists on a known-good computer, the device may need service.

If the issue is not resolved by USB isolation troubleshooting, and another computer is not available, try these steps to resolve the issue:

1. Connect the device to iTunes, confirm that the device is in Recovery Mode. If it's not in Recovery Mode,put it into Recovery Mode.
2. Restore and wait for the error.
3. When prompted, click OK.
4. Close and reopen iTunes while the device remains connected.
5. The device should now be recognized in Recovery Mode again.
6. Try to restore again.

If the steps above do not resolve the issue, try restoring using a known-good USB cable, computer, and network connection.

Error 1600, 1601, 1602

Follow the steps listed above for Error 1604. This error may also be resolved by disabling, deactivating, or uninstalling third-party security, antivirus, and firewall software. See steps in this article for details on troubleshooting security software.

This device is not eligible for the requested build (Also sometimes displayed as an "error 3194")

1. Update to the latest version of iTunes.
2. Third-party security software or router security settings can also cause this issue. To resolve this, followTroubleshooting security software issues.
3. Downgrading to a previous version of iOS is not supported. If you have installed software to performunauthorized modifications to your iOS device, that software may have redirected connections to the update server (gs.apple.com) within the Hosts file. Uninstall the unauthorized modification software from the computer.
4. Edit out the "gs.apple.com" redirect from your hosts file, and then restart the computer for the host file changes to take affect. For steps to edit the Hosts file and allow iTunes to communicate with the update server, see iTunes: Troubleshooting iTunes Store on your computer, iPhone, iPad, or iPod—follow steps under the heading Blocked by configuration (Mac OS X / Windows) > Rebuild network information > The hosts file may also be blocking the iTunes Store. If you do not uninstall the unauthorized modification software prior to editing the hosts file, that software may automatically modify the hosts file again on restart.
5. Avoid using an older or modified .ipsw file. Try moving the current .ipsw file (see Advanced Steps > Rename, move, or delete the iOS software file (.ipsw) below for file locations), or try restoring in a new user to ensure that iTunes downloads a new .ipsw.

Unable to contact the iOS software update server gs.apple.com

Error 1004, 1013, 1638, 3194: These errors may be the result of the connection to gs.apple.com being redirected or blocked. Follow these steps to resolve these errors:

1. Install the latest version of iTunes.
2. Check security software. Ensure that communication to gs.apple.com is allowed. Follow this article for assistance with security software. iTunes for Windows: Troubleshooting security software issues.
3. Check the hosts file. The restore will fail if there is an active entry to redirect gs.apple.com. Follow iTunes: Advanced iTunes Store troubleshooting to edit the hosts file or revert to a default hosts file. See section "Blocked by configuration: (Mac OS X/Windows) > Rebuild network information".
4. Try to restore from another known-good computer and network.
5. If the errors persist on another computer, the device may need service.

Error 20, 21, 23, 26, 28, 29, 34, 37

These errors typically occur when security software interferes with the restore and update process. FollowTroubleshooting security software issues to resolve this issue. In rare cases, these errors may be a hardware issue. If the errors persist on another computer, the device may need service.

All the above are from:

iTunes: Specific update-and-restore error messages and advanced troubleshooting