Why i get error “21” in iTunes ?

Why i get this error “21” in iTunes when i try to restore my iphone 3gs . why can explain me simple . help me please ….
Answer #1
Error 21 or error 1601 means there is something wrong with your USB connection or with the communication between iTunes and the iPhone.
P.s. (Sometimes error 21 means hardware failure/baseband chip, radio chip etc…)
P.s2 (Be sure that you are in DFU mode before restore process, use iReb to go in proper DFU)
Answer #2
ahh okay . . i put in dfu mode with run . because my sleep/wake don’t work for me
Answer #3
Donzz replied: ahh okay . . i put in dfu mode with run . because my sleep/wake don't work for me
Fix the damn button…
Answer #4
it cost wery expensive for me