My coolant temperature sensor just died. One symptom was an long run of electric fan after shutting off the engine, on a day that was not hot at all. (That only happened once.)

For what it's worth, my coolant temperature sensor had a greenish tinge on the outside, while all the rest of the engine looks pretty clean.

When it goes completely, you will get a check engine light, P0116 code, and possibly hard starting in certain temperature ranges. There is a copper crush washer that seals it against the thermostat housing, that does NOT come in the package with the sensor.