[LBinTX]

I have two T8601D2019 thermostats in my home that were installed new when the house was built in 2002. Recently I noticed the upstairs unit was displaying a temperature four degrees below what our program was set at. Our setting is "heat" with fan at "auto". With some fiddling around with the thermostat, I found that the flame icon would be on but the fan icon, nor the fan itself would actually come on. When I set the fan to "On", the fan icon would come on, as would the fan itself, blowing warm air. Later, I noticed that the thermostat would click between off and on, like it thinks it's working, but still no air out of the registers.
Does this sound like an issue with the thermostat, the furnace or something in-between? Thanks!