Honeywell TH9421C1004 blank screen after power outage

Make sure there is 24 Volts between the R & Common wires connected to the thermostat subbase.

The power on many of those units is derived from the unit itself. Check the control fuse on your furnace, or check the breaker, or switch, or other power source to the unit. But remember, there is a problem. Be very careful or you know what you are doing or you could burn your house down or hurt somebody.

check your float switch. On your air hander you have 3/4 white pvc coming out of bottom of unit, where the water drains out. If you have a float and your drain line is plugged it will make your tstat go blank. vacuum the drain line where it comes out of the ground outside.

It actually sounds like the batteries are going dead if the 24v from the furnace are not providing the power to the thermostat.

Possible defrost board. Get service tech to check this . Had the same problem change board it was ok. RUSS

Hi, take a look on amazon for replacement, here: Defy Mother Nature with our Best Buy. I hope this was helpful. Cheers

That thermostat must be used with a THM5421C1008 Equipment Interface Module. The thermostat communicates with the module and the module is connected to the HVAC components.