Wont charge or turn on? Thermometer on battery comes up?

Is the battery original or a replacement? If the battery is original (check for DOM printing), start there due to the age of the phone. A lot of these have batteries from 2015 and they’re all old enough cell resistance is becoming a problem. Consider yourself lucky you don’t have an Active; you need to ruin the $100+ display to change anything inside the phone. Don’t spend too much since a lot of the S6 batteries now are 90-95% health refurbs with some being 80-89% health refurbs, especially on eBay. It probably wouldn’t hurt to caution the customer of this possibility if you’re doing it for someone else.

IPA has water in it which doesn’t make it the best cleaner for water damage. The best options are denatured alcohol (poisoned Vodka without a sin tax) or methyl alcohol. I would go back and do it again to displace the water and board junk properly. IPA works, but you can do better. Follow these instructions to clean the board. An ultrasonic cleaner and denatured alcohol displacement is the absolute best option to do it, but I recognize not everyone has that luxury.

Refer to this guide to change the battery.

I guess I see why Samsung put that thermal sensor on the board.

The best way to fix this issue to replace the whole charging port board but there are other way to by pass this by removing the thermal sensor IC from the board if you need to have the phone to work ASAP.

But I wouldn't recommend doing it for long term solution as the sensor is there for purpose.