Also read: Top 10 Must Have Cydia Tweaks for iPhone, iPad, iPod touch
iReb will put your iPhone (also other iDevices) into a pwned (jailbreakable) state so that you can upload custom firmware to it.
It can be used to fix unknown error 1600, 1601, 1602, 1603, 1604, etc.
Download iREB RC4:
This compressed zip file contains iREB-r4.exe