What’s the ambient temperature, inside and out? (I know you said that the AC is on but on the very hottest days here, it can be that the AC struggles to cope, and it is baking outside and rising through the thirties °C inside.)
I’m not an electronics engineer so can’t directly answer the question about where and how the various relevant temperature sensors are implemented. You might have to ask Purism that.
If charging with the phone booted, you can check what the instantaneous charging current is, to make sure that it is a sane value (and perhaps you can clamp it if that helps prevent temperature rise).
Temperature inside is 75F/ 24C. Pretty close to what it has been for the past couple of summers that I have had the phone.
Instantaneous charging current is 0 A - because the phone thinks there’s something wrong it won’t even try. So I can’t lock it in really.
I had an extra battery, but hid it from myself so I can’t just swap quick to see if that’s the actual issue. And I don’t want to pay $60 for another one in case it’s just the chip gone bad. I’ll probably ask the Purism customer support whether the 0C error is coming from the chip or the battery. Unless one of them pops up and answers here in another day.