Lux
No 24VAC Power to Thermostat
Thermostat screen is completely blank and unit appears dead
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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 furnace control board, Disconnected R or C wire at Lux wall plate
How to Fix / Troubleshooting
Safety: You will be working near live circuits; turn off the furnace/air handler breaker before removing covers.
- Check breakers: Verify that the furnace/air handler and outdoor unit breakers are not tripped. Reset if necessary.
- Inspect furnace board fuse: Remove the furnace/air handler access panel and locate the control board. Look for a small automotive-style blade fuse (commonly 3–5A). Replace it with the same rating if blown.
- Check thermostat wiring: Remove the Lux thermostat and confirm the R and C wires are firmly connected and not broken. Re-strip and re-terminate if needed.
- Measure voltage: With power restored, use a multimeter to measure between R and C at the wall plate. If there is no 24VAC, the issue is with the transformer or control board and should be addressed by an HVAC technician.
- Replace thermostat: If 24VAC is present but the Lux thermostat remains blank, the thermostat itself is likely failed and should be replaced.
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Repair Difficulty
Medium
3/5
Required Part
HVAC Control Board Low-Voltage Fuse
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