iPhone 4 error 21 and will not start

if you already ruled out the battery for being defective, then proceed to the next suspect, which is the battery connector located on the logic board or mother board, try to check if its mounted properly, all the legs must be soldered properly and intact and clean. then if you ruled out the battery port or connector on your logic board and still having the same problem then proceed to the dock port or charging port (where you connect your sync cable or charger cable), try to clean it or better replace it. you can check in youtube the step by step procedure, if you already performed all these 3 things, and still having the same problem, try replacing the voltage regulator also located at the logic board, it looks like a tiny black color square shape thing, youtube can help you locate it. you need a very good skills of replacing this part, all of these steps are inexpensive repair, but like i said, you need a good troubleshooting and soldering skills to be able to perform those repairs, if you did all those and the problem still persist, it must have a serious mother board problem, the phone maybe water damaged before, you cant assure anything if the phone dives in the water, just tryin to help.

I know this is a long time later, but people still read these things.

I fixed a phone with an error 21 behaving the same as the original post. Upon closer inspection I found one of the tiny wires/pins on the battery connector was bent. A gentle tap and it was all lined up for the battery to plug into.

Problem solved.

Worth a try for a cheap fix.

Hello Cesar,

I think your iphones battery connector is broken, open the back cover and remove the battery and check the battery connector .

any luck ?

press home and power button at same time wait for apple logo release , if still dosnt work change battery and usb port