This time around when the 'F8' error (LG/Westinghouse) showed up I removed the sensor and heated it to 200C with hot air. Perhaps 2 minutes was enough.
Testing with an ohmmeter showed the 'cool' resistance to be back to 5K ohms (up from 137 Ω when removed from the a/c).
So, I guess the moisture seal fails and then moisture & corrosion screws the thermistor function.
Before replacing I sanded off the green & black corrosion and coated with petroleum jelly.