Temperature problem after water damage

Hello from germany, I bought an iPhone 4 with an waterdamage to test something around. After I changed the batterie, it starts normaly. The battery was nearly empty, so I let it charge over night. On the next morning I activate it. During the activation, the phone shows me the following error:"iPhone needs to cool down, before you can use it".

I only can shut it down or make an emergency call.

But the phone is not warm or hot. After 1 hour in the fridge (+8°C) I started the phone again. Now I know that the home button is faulty and makes no reaction.

I take a look in the Crash Reports but I don't understand it. I can filter out a line with "thermal sensor level 15" and after this, there are many lines with a lot of paths.

I have no apps on the phone, it's resetet.

Can anyone help me? I can't find anything about an temperature sensor in the iPhone 4.

I can't imagine that