iPhone displays high temperature error after iOS27 update despite being cold

The phone might be hot internally around the processor from the activity to load and install the new version. I suggest remove any third party case to allow better heat dispersal, and put the phone aside to let it cool for at least 30 minutes. If that doesn’t solve the problem then power down for another half hour before rebooting.

It might be a bug in 27 because there have been a number of similar reports. I suggest that you give feeback to Apple; the more reports they get the more likely they are to fix it.

Product Feedback - Apple

US575 wrote:

The device displays an error indicating high temperature,

Not every issue an individual encounters on their device can be attributed to an iOS update. iOS 27 or later may have coincided with what you are experiencing, but that does not mean the issue was triggered by iOS. Coincidence in timing is not, by itself, evidence of causation.

If the update itself were the root cause, then every device running that same version of iOS would exhibit the identical problem. In such a scenario, the forums would be flooded with countless posts reporting the same malfunction, creating an unmistakable wave of complaints that no one could possibly overlook.

The absence of such widespread reports strongly indicates that the issue is not a general, system-wide problem with iOS 27 or later, but rather something specific to the individual device, its configuration, installed apps, settings, hardware, or another interacting factor.

In short: iOS 27 or later may be a coincidence in timing, but there is no basis to conclude that iOS itself triggered the problem simply because the issue appeared after the update.

See what each update does: About iOS 27 Updates, About iOS 26 Updates, About iOS 18 Updates, About iOS 17 Updates, About iOS 16 Updates, About iOS 15 Updates, About iOS 14 Updates, About iOS 13 Updates, About iOS 12 Updates, and so on...