Weather Factors

• According to Apple, the iPhone 3GS has an optimal storage temperature range of -4 to 113 degrees Fahrenheit. Users are instructed to operate the mobile phones in temperatures not exceeding 95 degrees Fahrenheit. Operation in environments above this temperature affects the internal workings of the iPhone 3GS, causing the phone to overheat.
  
  

App Usage

• CPU-intensive applications, such as GPS navigators and iPod functions, also affect the operating temperature of the iPhone 3GS. This is especially true when using such apps in direct sunlight or in a warm vehicle. If your iPhone has been left is a heated area for a long period of time, it is best to allow the device to cool before attempting to operate.
  
  

Original Firmware

• A corrupt image in the phone's original firmware (3.0 firmware) caused the phone to operate at 100 percent, even when in standby, which contributed to overheating. However, due to numerous firmware updates since the phone's release, it is likely most 3GSs are no longer running the old firmware. If you find that your phone is, use iTunes to do an update on the device.
  
  

Charging Battery

• A support document from Apple states that the exterior of the iPhone 3GS acts as a cooling surface, transferring heat from inside the phone to the cooler air on the outside. When charging the battery of your phone, it is completely normal for the interior to heat up, thus causing the exterior to become warm. This heating of the iPhone will not cause damage or loss of functionality in the device.
  
  

Effects of Overheating

• If your iPhone 3GS begins to overheat you will notice effects such as the device stops charging, the display becomes dim and the device's location services become disabled. The 3GS also prompts a warning screen stating that your "iPhone needs to cool down before you can use it." Exposure to excessive temperatures may also decrease the phone's battery life or cause the phone to completely stop working.