If you get a 1600 or 16xx error while trying to restore to a custom 3.1 firmware for iPhone 2G or 3G, try the following :
Try this first instead of entering DFU mode…for 3G
1)UNPLUG ur iphone and turn off
2)Open itunes
3) hold home as u plug in iphone and keep holding until u see itunes logo on ur iphone
4) then SHIFT+restore and locate the custom ipsw
and there u go! (thanks txccu2000).
If that didn’t work, then try the following :

Try rebooting both iPhone and PC/Mac.
Make sure the iPhone is in DFU mode.
On …