Chosen Solution
After an ios update my iphone 6s got hot, the battery died and would not power on. This was months ago and I do not know which version it updated to. I decided to try replacing the battery myself and it powered right on after the change. Charged it overnight to 100%, It would not connect to my WiFi though, kept telling me the the password was wrong. Got an error when trying to share the password via another iphone. Opened iTunes on my laptop and tried updating but got “unknown error”. Same with restore. Phone heated up quite a bit through all this, top of phone seems hottest. It won’t power on on. When trying a hard reset the Apple logo screen comes on and then the screen goes black right away.
Connecting to laptop and iTunes now tells me phone has to be updated or restored. Updating leads to “unknown error (75)" and the phone stays stuck on the Apple logo screen and heats up. Disconnecting leads to black screen. Plugging into charger there is nothing on the screen.
At one point when trying to connect to my WiFi it gave a message that said to try to move closer (to the router?) but that didn’t help, neither did resetting the network settings. There was no option for me to forget the network.
My guess is the updated version is frying the battery but I wonder if there is another hardware issue I should look into. Any suggestions for what to do now?
It sounds like there is a short in your Wi-Fi IC this is common on the 6s and produces some of your symptoms. I have only seen error 75 while updating not restoring but I would imagine it has something to do with the short in the Wi-Fi IC. You could try digging the short out or replace the IC both would need microsoldering skills and a NAND programmer if the Wi-Fi IC is replaced.
