When your iPhone overheats -- which can happen, for example, if you're using GPS in direct sunlight or even if you just accidentally leave your iPhone in your car on a hot day -- it will display a yellow caution sign on the screen and alert you to the fact that the internal temperature is too high. It will also attempt to cool itself down by performing a number of heat-reducing steps -- including dimming the screen and stopping the current charging session.
What Do You Do Now?
There are a number of ways you can help the device cool down faster. First, unplug it from the charger, then power it down by holding the Power button for a few moments. If possible, move the iPhone to a cooler environment -- like an air-conditioned room or, if outside, into the shade. You can use it again once the device reaches a safer temperature -- at which point it will clear the error and caution sign from the screen.
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