Radiator outlet taped probe. If I set the thermostat to turn on when the temp is in the middle of the arc, the fan runs for more than 30 minutes until the temp sensor drops to COLD! When the temperature is a little higher, say on the way to overheating, the fan runs just above cold! I will try different probe placements. The thermostat is running the fan for TOO long! It stands on a single-row plastic radiator tank made of aluminum.