How can I diagnose an overheating iPhone 14 Pro Max?

As Bob Timmons correctly suggests, if you didn't see a red thermometer with a message it can't be used until it cools down, it didn't overheat. That doesn't mean it didn't get warm or even quite warm, but then we don't know what you were doing on your phone when it heated.

One of the biggest reasons an iPhone battery becomes warm is its health. If you open Settings > Battery > Battery Health - it is at 80% or lower, the useful life of the battery had been spent and it needs to be replaced. Depleted batteries have to work harder to run your phone and when batteries work harder, the generate heat naturally.

When you say that the phone is overheating, this means that you are seeing the message below on your iPhone....correct?

Diagnostics are done at the Apple Store / Genius Bar or at an authorized Apple service shop. You likely have a defective battery or a problem with the main circuit board.....or both.

To confirm, you can find a list of authorized shops in your area using this link:

Find Locations

Good luck.