Thermostats for the furnace to heat the house showing er

Mary, are you sure the thermostat is in fact getting the furnace to start...???? It sounds like the furnace is NOT starting in heat mode.

Since you have two thermostats that operate the heating unit and the fan, and the fact that neither will turn the blower on, I would suspect the furnace/ air handler. Turn the thermostat to heat and see if the heat and the fan comes on, if it does operate normally in heat, the fan relay or the high speed on the blower is most likely the culprit. (If the furnace is equipped with a circuit board, the fan relay is integral to the board, if it an older furnace it has a separate fan relay). If it does not operate normally in the heating mode, verify power to the furnace, see if the flame comes on, if it does: the fan motor, circuit board or the fan relay are most likely the cause. hope this helps!

Check your fuses and your pilot light on the furnace itself

did you push the little reset button on thermostat?

Sounds as though another problem is showing up. Verify T-stat is sending the red 24volts (alt current) through the White wire. This can be done at the t-stat or the furnace whichever is easier to access. At t-stat your meter will show 24volts from red to any wire that the t-stat is not jumping to. So if you put on lead on Red and other on White and 24 shows up the t-stat is not connecting that and the furnace will (NOT) turn on heat. If you get 24 on Y or G but nothing on White then the t-stat is (Good) and the furnace in not kicking on.
To diagnose furnace look for flashing light on board. count flashes and read furnace diagram for info. Furnace diagnosis will take some patience and observation of what happens when. Problems can be Airflow, or parts. It takes some watching. I've found tools jammed in vent stacks, blocked (dirty) evaporators and failing sensors or relays. All causing the units to start but not stay lit.

Do you have a water heating system?

I have the same issue with a G21Q4-5-100-1 model lennox. House drops 8 - 10 degrees. I have 3 thermostats and all react the same way. I was told by service tech that the Ignition Coontrol Module is bad and acting intermittanely. ???

fuanace may be oversized and is short cycling it heats up house to fast it is better when they run longer

something is wrong with the thermostat.  replace it first, that would be the cheapest alternative

Sounds like a relay sticking. Possibly in the thermostat or possibly the furnace relay at the furnace. The next time it happens, unplug the thermostat from the wall. If the furnace still runs then it's the furnace relay.

Hope that helps...

Geno