HeatingCodes

Thermostat screen blank or not powering up: what to check

By Jason Dearmin · Last reviewed November 14, 2026 · How we check facts

Short answerHoneywell Home's support pages list the same first checks for a blank display: the circuit breaker, the power switch for the heating and cooling system, the furnace or air handler door closed securely, and a connected C wire. Only the first three are routine homeowner checks.

Quick checks (no tools)

  1. Check the circuit breaker for the furnace and reset it if necessary.
  2. Make sure the power switch for the heating and cooling system is on.
  3. Make sure the furnace or air handler door is closed securely. (Carrier notes that removing the blower door cuts 115 V power to its furnace control.)
  4. If the thermostat uses batteries, replace them with fresh ones. The Honeywell T4 Pro manual covers battery replacement for that model.

Check the C wire

Honeywell explains that its smart thermostats operate on 20 to 30 VAC supplied to the R or RC terminals and returned through the C terminal, and that a C wire connected to C on the equipment is required for the thermostat to power itself. If your previous thermostat did not use a C wire, it is unlikely to be connected at the equipment. For T9 and T10 models, Honeywell says that if no wire is in the C terminal you should use the C-wire adapter described in the installation guide.

Reset the thermostat

For T9 and T10 thermostats, Honeywell describes two resets: gently pull the thermostat off the wall near the base, wait one minute and put it back, or turn the service switch on the furnace or AC system off for 30 seconds. It says the device should recover within about 10 minutes.

The jumper switch and voltage test

Honeywell also says to check the jumper selector: with a wire in R and none in RC the switch should be in the 1 Wire position; with wires in both R and Rc it must be in the 2 Wire position. Measuring AC voltage between R/Rc and C with a multimeter is listed as an advanced step; readings outside 20 to 30 VAC should be addressed by a professional contractor.

Safety Turn off power at the furnace switch or breaker before touching thermostat wiring or the furnace control. If you are not comfortable with low-voltage wiring, call an HVAC technician or Resideo Customer Care at 1-800-633-3991 (Honeywell Home support).

Fix a blank thermostat screen

  1. Check the breakerReset the furnace breaker if it has tripped.
  2. Check the furnace power switchMake sure the heating and cooling system power switch is on.
  3. Close the furnace doorMake sure the furnace or air handler door is closed securely.
  4. Replace batteriesIf your model uses batteries, install fresh ones.
  5. Reset the thermostatPull it from the wall base for one minute and replace it, or turn the system service switch off for 30 seconds (Honeywell T9/T10 guidance).
  6. Call a professional if still blankA missing C wire or voltage problem needs a technician or Honeywell support.

Frequently asked questions

Why did my thermostat go blank after a power outage?

Check the breaker, the furnace power switch and the furnace door first. Honeywell says T9/T10 units should recover within about 10 minutes after a reset.

Do all thermostats need a C wire?

Honeywell says its smart thermostats do. Basic battery-powered models do not. Check your model's manual.

Sources

Related

Furnace after a winter power outageBest thermostat settings for winter savings (and the heat pump exception)

Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.