That's the sensor on the roof. It also means that the system has gone into freeze-protection mode. Check the wiring from the Solar Touch to it. Sometimes the insulation wears off, or rodent damage (they love to eat the stuff), and the conductors short. Its an easy replacement if you are comfortable getting on a roof. Any brand temperature sensor will work, they are not specifically for solar, water, or air temperature. Even the inexpensive ones on Amazon from Wholesale Sensors, at 2/$15.00, seem to last.
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