My iPhone 3GS started crashing on me after upgrading my iOS in Dec 2011. The battery percentage dropped quickly, but then randomly when trying to use facebook or google maps, my phone would just shut off. I would have to turn it back on and try again, sometimes going through this process 3 times to actually get the information I was looking for. It was beyond frustrating and felt as if I had a non-functioning phone.
I was very nervous when I started these steps, I haven't done a lot of technical things with computers in a long time so I wasn't sure I could even do it without breaking something. Thankfully I'm here to say that I was able to do it, without breaking anything (so far) and my phone is actually working again (so far).
I did run into a couple problems, the first one was getting the battery out, it was seriously stuck in there. I broke one end of the plastic spudger and then switched to some tweezers to finally get it to come off. I swear I was going to break the back of the phone any second though trying to pry it out. Very frustrating.
Second, when trying to put the logic board back in I struggled to get it into the proper place for all the screws to line up and be pushed far enough in to actually get the screws to go in. Eventually I started just pressing it down and screwing them in and it sort of popped into place. I was afraid I broke something but it seemed to be ok.
Finally, that darn number 6 cable, it seriously took me a solid 20 minutes to get that one reconnected. I almost gave up and was going to live without gps because I could not get it to snap on. Then all of a sudden when I was pushing down, I let up and it didn't pop off. I tested it a little and it seemed to be in place, so even though it didn't make a "snap" sound, it finally connected. Whew!
I read the paperwork and watched the video several times before I got started. The only thing I didn't understand, and still don't, was one step in the doc that said to attach the camera to the logic board before putting the logic board back into the phone. Huh? It attaches to the phone via screw and remained attached to the logic board when I took it out... so I don't get that instruction at all.
Other than that, everything was well documented and besides the few things I had trouble with, which had nothing to do with the instructions, it went pretty well.