Short answer
A blank thermostat is usually dead batteries or no power from the system. Replace the batteries first. If that doesn't help, check the breaker and power switch for the indoor unit, and look for a full drain pan, because a tripped float switch can cut power to the thermostat. If the screen is still dark after those, call.
What's probably happening
A thermostat is a small switch on the wall. It gets its power either from batteries or from a low-voltage transformer inside your furnace or air handler. When the screen goes dark, one of those power sources is gone.
Dead batteries
The simplest answer and a common one. Some thermostats warn you for weeks, and some just quit.
The indoor unit has no power
If the furnace or air handler loses power from a tripped breaker or a switched-off power switch, a thermostat that runs on system power goes blank with it.
A tripped float switch
On many systems the drain safety switch is wired so that a full drain pan cuts power to the thermostat. The screen goes dark and the system stops. That's on purpose. It's keeping water off your ceiling.
A blown fuse on the control board
Many furnaces and air handlers have a small fuse on the control board. A short in the thermostat wiring can blow it. Finding what caused the short is a job for a technician.
A failed transformer or thermostat
Transformers and thermostats do wear out. If everything else checks out, one of those is likely.
What you can check yourself in five minutes
- Replace the batteries. Most thermostats pull straight off the wall plate or have a small battery door. Use fresh batteries of the type it takes.
- Check the breaker panel for the furnace or air handler, and for the outdoor unit. Reset a tripped breaker once.
- Find the power switch on or near the furnace or air handler and make sure it's on.
- Check that the furnace or air handler doors are closed and seated. A door switch can cut power.
- Look at the drain pan under the indoor unit and any float switch on the drain line. If the pan is full, the float has likely tripped. See the water leak page.
- If you have a smart thermostat, check its app for an error or a power warning.
If the screen comes back and the system runs normally, you're done. If it came back because you cleared a full drain pan, the drain itself still needs attention, or the same thing will happen again in a week or two.
One more thing worth knowing: if the thermostat was recently replaced and has never worked right since, the problem may be the wiring or setup, not a failure. Heat pump systems in particular need the thermostat set up for a heat pump, or the heat and aux heat won't behave.
When to stop and call
- The breaker trips again after one reset.
- You smell electrical burning near the thermostat, the furnace, or the air handler. Turn the system off at the breaker.
- Water is on or near the indoor unit's wiring. Shut the power off first.
- You smell gas. Leave the house and call your gas utility or 911 from outside.
- The screen stays dark after new batteries and a power check, or it comes back and goes dark again.
Please don't pull the control panel off the furnace to look for a fuse. If you're in Spartanburg or Greenville County, call Alpine Cool at (864) 702-0777.
Common questions
My thermostat doesn't use batteries. Why is it blank?
Many thermostats run on low-voltage power from the HVAC system itself. If the system loses power, trips its float switch, or blows the small fuse on its control board, the thermostat goes dark too.
Why did my thermostat go blank right after the AC started leaking?
On many systems the float switch cuts power to the thermostat circuit when the drain pan fills with water. Clearing the drain usually brings the screen back. Don't bypass the float switch.
Can I replace the thermostat myself?
Some people do. But heat pump thermostats have extra wires and settings, and a thermostat set up the wrong way can run aux heat far too often. If you're not sure, have it set up for your system.
The screen came back after new batteries but the system still won't run. Now what?
Check the mode and setpoint, then the breakers and the furnace or air handler power switch. If those are fine, the problem is past the thermostat, and it's time to call.