"1600" error when attempting to downgrade

[MatrixPrime]

I've been trying to follow the tutorials to downgrade my 3G back to 3.0.1, but in iTunes, I never get the 1510 error, I get a 1600 error. What do I need to do at that point?

 

[dZp]

On a Mac ?
Delete the contents (there should be a small .ipsw file) of the following folder:
~/Library/iTunes/Device Support/

~ = your user directory

And try again. Worked for me anytime I got error 1600 trying to restore to a previous firmware.