Somehow the long answer I made failed to make it. On the PA board there are thermistors that read the temp. I think that they are R734 and R736 The hottest is directed to the CPU via D705 as the THML signal.
The CPU develops the temp meter on the display. These thermistors are fed from the +8 volt supply IC701.
On the schematic PA unit (1) upper left.
A quick search will find several articles on the heat and one has pictures of the thermal compound being added to get a better thermal connection of the thermistor to the heat sink.
The CPU (IIRC) also develops the FAN signal to turn on the fan. Some have modified the circuit bypassing the on off to have the fan run continuously.
When running FT8 I never ran over 50 watts, usually 30. A good antenna is a great addition to the station for FT8 or really any mode. I worked the world with a 43' vertical and a good radial system.