What does the red light on my thermostat mean?
If the thermostat is in Emergency Heat mode, the red light is normal and simply indicates the thermostat is operating in that mode. If the thermostat is NOT in Emergency Heat mode and the red light is on, you should contact a qualified service contractor for repair. (Page 66)
From the official Honeywell RET97B5D100 Home Programmable Thermostat manual, page 66
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