iPhone4 won't restore and shows Error 21

you don't have to replace battery or even take it out just take off back disconnect the battery(one screw) i don't know if you need to or not but i held home button down for 30 seconds to help fully reset the phone then push the silver connector back on and put phone together. now hold down home button and same time plug it into computer and keep holding home button until phone shows to plug in to iTunes so make sure iTunes is already open before plugging in. then do complete restore and you'll be back in business. trust me i tried for two days i thought i had a paper weight.

"I have had this problem for the past 2 days and I finally fixed it. It had nothing to do with the system but the the connection between the battery and the phone had gotten messed up. It kept trying to restore but only ended up with error 21.

My solution: I opened the phone, removed the battery, cleaned out the battery connector and reattached the battery. The phone turned back on and showed the low battery sign. I had done this before and it didn't work. Apparently the problem was I hadn't properly connected them. You need to make sure it's locked on tight so when you close your phone slides on easily. I hope this can help someone else. I was ready to buy a new battery and now I don't have to."

i used to have same problem,apple logo apear and desapear, when was connected to a wall charger or to pc.the problem was the battery it doesnt hold charge then the phone cannot load the os or can not be restored cus phone is no charged replacing the battery solve that issue hope this help to you.

I have had this problem and as a trial swapped battery's with an identical iPhone 4 the phone worked fine then, I then put the old battery into the other phone and the error/ unable to boot up occurred so a new battery will resolve error 21 or other similar boot up problems and is a cheap fix £2.50-£3.50 on e-bay.

"I faced Error 21 recently and the only way I was able to solve it was by removing the battery and leaving the phone for 2 minutes. I put the battery back, connected the phone to iTunes and the restore worked like a charm.

PS: I am guessing that all the phone needed was a complete hard reset."