What is this red LED light on my Aprilaire 600 humidifier?

Wednesday, December 7, 2011 by Aprilaire Team
Consumer question:

Solenoid valve does not appear to open. Shut down furnace. Changed water panel. Adjusted furnace to call for heat. With blower on, about 1 minute, green "change water panel" light and red LED inside the control panel, both come on simultaneously. After about 1 second green light goes off. Red LED, marked D10, on the circuit board remains on. Moved control knob toreset/test position for one minute, and adjusted to previous "normal" humidification setting. Red LED stays on, and no water is being fed into evaporator. -Water to solenoid is "on." Any suggestions??

Aprilaires' response:

Thank you for your email regarding your model 600 Aprilaire humidifier. The red LED indicates the outdoor temperature terminals(ODT) are not receiving a temperature reading from the outdoor temperature sensor. The humidifier control will not function without the outdoor temperature reading. Our recommendation is to have your HVAC installer review the wiring and test the outdoor sensor and replace sensor if necessary.

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