No Power to Thermostat
Honeywell Home
Loss of 24VAC Control Power
Honeywell Home smart thermostat screen is blank or dark and unresponsive
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Tripped furnace/air handler breaker, Blown low‑voltage fuse on control board, Disconnected R or C wire at thermostat, Failed 24VAC transformer
How to Fix / Troubleshooting
Safety: Turn off power at the breaker before removing equipment panels or touching wiring.
- Check breakers and switches: Verify the furnace/air handler breaker is on and the furnace service switch is in the ON position.
- Inspect thermostat wiring: Remove the thermostat from the wall plate. Confirm that R and C wires are firmly connected at the wall plate and not broken or corroded.
- Check furnace control board fuse: Remove the blower door and locate the low‑voltage fuse (often 3A or 5A automotive style) on the Honeywell or OEM control board. Replace if blown, but also inspect for shorts in thermostat wiring.
- Measure voltage: With a multimeter and power on, measure between R and C at the furnace board. If no 24VAC is present, the transformer may be failed and should be replaced by a technician.
- Reinstall thermostat: After restoring power and correcting wiring, snap the thermostat back onto the wall plate and verify that it powers up.
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Repair Difficulty
Hard
4/5
Required Part
24VAC Furnace/Air Handler Transformer
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