Can an overheated S23 Ultra be repaired?

@larry34658 that is a depends question. Depends on what, if anything , got damaged by the overheating. You did not tell us what "Geek Squad" suggested to you or where the "humidity" is located. If anything that will need to be handled like a water damaged phone. Let's start there.

Here are the videos I suggest for you to use as a guide for your teardown. you will need to get the mainboard as well as the charging board out. Video one and here is video 2

For now, you must stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse. This may further damage your phone.

Disassemble your phone using the videos linked to. Remove the main board and the charging board. You will have to remove all the EMI shields. If you do not remove the shields, you are not cleaning the board.

Take a careful and close look at all components and connectors. Truly assess the damage. Look for major corrosion, evaluate the pins in the connectors etc,. Take a look at all connectors as well as cable ends. For a proper cleaning, use this guide. It was written for an Apple iPhone 3G but all the steps are still pertinent to your phone as well. I can not stress the importance of a good cleaning enough, so do it over and over while replacing the alcohol after each cleaning.

Do not use compressed air for the cleaning since that can drive liquid as well as corrosion particles into areas that will cause trouble later on. You do not need to leave it to dry for a day. Isopropyl alcohol in the higher concentration will evaporate quickly and dissipate the water.

The importance here is the cleaning. It would of course be best to get it ultrasonically cleaned

Now you took care of the "humidity" issue and did an inspection of your boards. Next is to reassemble your phone. Make sure that you checked the ribbon cables for corrosion etc. as well. You MUST install a new battery for this since one of the thermal sensors is on it and your original one may have failed. I would also consider to replace the charging board to make sure thee are no issues with that. However, that is personal preference at this time.

If it still overheats, you do want to check the components either by some sort of thermal imaging or you can use some freeze spray. Apply the spray and then connect a power source to your phone and turn it on. Watch the melting pattern of the freezing spray and you will find the component that is overheating. You would now need a component layout and the schematic for your phone to determine which part you need to replace.

I am sure there are a few details that I accidentally omitted but essentially that is how I would proceed. There are many different ways of doing this and everybody develops their own technique. There is no wrong or right way, for as long as it works to get your phone working again.

While you work on your drive, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!