I have got loads of emails and queries via comments on the posts here and here where many of you have been asking on how to fix 1604, 1603, 1602, 1601, 1600 error in iTunes during firmware restore. This error is occurred when you are trying to restore your jailbroken custom iPhone 3.1.3 .ipsw file which you cooked using either PwnageTool or sn0wbreeze.
When trying to restore your iPhone or iPod touch in iTunes with iPhone 3.1 firmware, you may get an error message followed by an error code 1601, 1602, 16xx or 21. To get around this problem, iH8Sn0w has released a handy little tool called iREB (iTunes Restore Error Bypasser) for both Windows and Macs which can help bring your iPhone back to life. The latest version of this program supports iPhone 3.1 firmware running on the original iPhone, iPhone 3G and iPod touch 1G.