E8
Lux
Low 24VAC Supply Voltage
Thermostat displays E8 and backlight may flicker or dim
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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
Weak HVAC control transformer, Loose R or C wire at wall plate, Excessive voltage drop on long thermostat cable
How to Fix / Troubleshooting
Safety: Turn off power to the HVAC system before touching wiring. Use caution when measuring live low-voltage circuits.
- Inspect wiring: Remove the thermostat and check that the R and C wires are firmly clamped in their terminals on the Lux wall plate. Re-strip and re-terminate any corroded or frayed conductors.
- Measure voltage: Restore power and use a multimeter to measure AC voltage between R and C. It should be around 24VAC. If it is significantly lower (e.g., under 18VAC), the transformer or wiring is suspect.
- Check at air handler: Have access to the furnace/air handler control board? Measure 24VAC directly at the transformer secondary. If voltage is normal there but low at the thermostat, the cable run is causing drop or has damage.
- Correct wiring issues: Tighten low-voltage connections at the furnace control board. Replace damaged thermostat cable if needed.
- Replace transformer: If voltage is low at the transformer itself, an HVAC technician should replace the 24VAC control transformer.
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Repair Difficulty
Hard
4/5
Required Part
HVAC 24VAC Control Transformer
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