What temperature is the device?

I'm not sure which type of overheating you are referring too. During normal use any electronic damage inside the phone can draw battery power to it and cause it to over heat. If its bad enough then the device will shut itself off and turn back on. The screen will go black and then you see the Apple logo. This is because a sensor in the phone is causing the phone to shut off. Then it restarts itself and it will try again. If the boot loop is long enough you can get the crash log and it will give you the codes for why the phone is rebooting. That will indicate if you need a particular part replaced like the poximity sensor or the charging port.

You will get a temperature warning if the device temperature is over 95 degrees Fahrenheit. The phone will tell you it is overheated and it has shut down to let the device cool. This is caused from being in the sun, a hot car, or anything that will cause the overall temperature of the phone to increase.