This one needs a licensed pro
Likely cause: wiring, a C wire, or a failed thermostat
With batteries, power, the drain and the base all ruled out, the problem is in the wiring or the thermostat itself. Call an HVAC technician.
Why this happens
Honeywell Home says a C wire is required for many thermostats to power themselves, and a thermostat that went blank after a swap often lacks one. Honeywell also lists a missing or faulty C wire among the causes of a blank screen and says voltage readings outside the normal 20 to 30 volt range should be addressed by a professional contractor. Trane lists loose wiring among the causes.
What to do right now
- Leave the thermostat wiring alone.
- If the house is getting uncomfortable, set the furnace or air handler switch to off so the system doesn't run without control.
- Note whether the thermostat was recently replaced or the system recently serviced.
- Call an HVAC technician.
What to tell the HVAC technician
- That the screen stays blank with fresh batteries, power on, door seated and no water in the pan.
- Whether it went blank after a new thermostat was installed, and whether that thermostat uses a C wire.
- The thermostat brand and model, and the furnace or air handler brand.
Build the full list: what to tell the HVAC technician.
Start this troubleshooter over
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