iTunes Error 1603 Preventing iPhone 3GS to Restore

Hello Paolo. Well, this error (iTunes 1604,1603,1602,1601 and 1600 errors) normally occurs when downgrading or restoring a custom IPSW firmware file into an iPhone if the users were trying to restore it using Sn0wbreeze or PwnageTool and the regular DFU mode. Anyway, to fix this error, you can use the new iREB RC4 tool. If you aren’t familiar with the steps, then you can follow the step by step guide posted below:

Step 1: First, you will need to download iREB RC4 and save it on your desktop.
Step 2: Connect your iPhone 4 into your PC via USB cable and launch the downloaded iREB-r4.exe.
Step 3: Then put your iOS device in DFU mode:

• Press Power and Home button at the same time for 10 seconds
• Release the Power after 10 seconds but continue holding the Home button

Step 4: iREB will soon start injecting the Limera1n exploit in the iBoot.
Step 5: Once completed, iREB will inform you that your iPhone is in Pwned DFU mode.
Step 6: Then wait for at least 10 seconds or so and launch iTunes.
Step 7:  Once iTunes is opened, select your device at the left side panel. Make sure that you are pressing SHIFT while clicking on the Restore button and browse for the custom iOS 4.3.3 IPSW file and hit OK to commence the restoration process. If you have encountered error 16xx again, immediately disconnect your iPhone and perform the entire process.

You can check this blog post for detailed tutorial and screenshots. Mac users can use Redsn0w as iREB’s alternative.