If your phone originally came with a battery part number apn 616-0433 and have since replaced your battery with apn 616-0431 you may find that you always get an error 29 when upgrading to ios 4.x Some people have reported that apn616-435 also works as a replacement battery and doesn't cause an error 29. Most vendors, including ifixit.com ship 0431 as their 3gs battery part.