A few weeks ago I decided to replace the battery of my iPhone 4S because it barely lasted half a day. Furthermore it simply shut off at a battery level of 20% which couldn't even be fixed by calibrating it. The replacement instructions at iFixit seemed pretty straightforward containing only seven simple steps. I was glad that iFixit has a webshop in Europe where I bought a battery replacement kit including the necessary tools. As far as I know iFixit sells only OEM parts which is great because I didn't want a cheap (and probably low-quality) replacement battery.
Opening the case was easy. After removing two screws next to the dock connector the rear panel could be pushed away.
The battery itself was adhered to the case and had a connector that was screwed to the mainboard. After removing the screws the battery could be pried up with the plastic tool. Inserting the new battery and following the guide in reverse order finished the job.
The iPhone could be turned on without a problem and had a battery charge of 75%. But wait.. The signal stayed at "Searching...". Did I destroy the antenna? After disassembling and reassembling the battery several times nothing changed. I remembered the warning in the guide:
"When reinstalling the pressure contact, be sure to clean it with a degreaser such as windex or isopropyl alcohol. The oils on your fingers have the potential to create wireless interference."
So I bought isopropyl alcohol and cleaned every piece of the connector, the screws holding it in place and the pressure contact grounding the antenna. No change and still "Searching...". There were strange comments below the guide at iFixit recommending to turn WiFi on which solved the problem for some people. Well, WiFi worked indeed but did not fix the GSM problem. Pascal had a great idea: Setting the date and time! He was right, with the date 1970-01-01 my phone could not connect to the carrier. After synchronizing the time and rebooting the phone everything worked as expected.
(Thanks to Pascal for taking the photos and troubleshoot. )