Thermostat blank: batteries, breaker, then the safety switches
A blank thermostat is usually lost power, and the two homeowner checks take five minutes. If the breaker has tripped, Trane advises calling rather than resetting it yourself, because something upstream caused the trip.
What is probably happening
- Dead batteries, on models that use them.
- The furnace or air handler lost power: its breaker tripped, the light-switch style power switch on the furnace got knocked off, or the blower door is not fully closed and the door safety switch is open.
- A full condensate drain pan tripped the float switch, which cuts power to the thermostat on many systems. Look for water at the indoor unit.
- A blown 3 or 5 amp fuse on the furnace control board, often from a shorted thermostat wire.
Do this in the next five minutes
- Pull the thermostat off its base and replace the batteries if it has them.
- Find the switch on or near the furnace that looks like a light switch. Make sure it is ON.
- Check the furnace or air handler breaker. If it is tripped, leave it and call.
- Look at the indoor unit for water in the pan or on the floor. If there is, use the water path on this site: the float switch is doing its job.
- Make sure the blower compartment door is pushed fully closed.
Do not
- Do not pull thermostat wires or bridge terminals to test. A short there blows the board fuse.
- Do not reset a tripped breaker repeatedly.
Who to call
A licensed HVAC technician
The control board, its fuse, the float switch and the low-voltage wiring are HVAC work. Only if the panel breaker itself is faulty does it become an electrician's job.
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What it should cost
$150 to $350
Thermostat replacement, national range. A control board is $100 to $600 if the fault is there
Source: Angi, HVAC repair cost, updated August 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Was it the fuse, the float switch, the transformer or the board? Can you show me?
- If the fuse blew, did you find the short that blew it?
- If you recommend a new thermostat, is it compatible with my system's stages and heat pump wiring?
- Does the service fee come off the repair?
End the call if
- A smart thermostat upsell as the fix for a blown fuse or a full drain pan.
- A board replacement with no test of the transformer output first.