E9
Lux
C‑Wire Power Loss or Insufficient Power
Thermostat displays E9 and will not power up fully or reboots when calling for heat/cool
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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
Disconnected C wire at furnace board, C wire not connected at thermostat wall plate, Shared power-stealing configuration incompatible with Lux model
How to Fix / Troubleshooting
Safety: Turn off HVAC power at the breaker before working on low-voltage wiring.
- Verify C at thermostat: Remove the Lux thermostat and confirm a wire is connected to the C terminal on the wall plate. If not, a dedicated C wire or Lux power adapter is required.
- Check furnace board: At the furnace/air handler control board, confirm the same color wire is connected to the C terminal and that it is secure.
- Continuity test: With power off, disconnect the C wire at both ends and test continuity end-to-end using a multimeter. Replace the cable if open.
- Install power adapter (if needed): If no spare conductor is available for C, install a Lux-compatible 24VAC power adapter or C‑wire adapter kit according to its instructions.
- Re-test: Restore power and confirm the thermostat boots without E9 and remains stable when calling for heat or cool.
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Repair Difficulty
Medium
3/5
Required Part
C‑Wire (Common) Thermostat Cable or Lux C‑Wire Adapter
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