E0
Armstrong Air
Communication Error Between Thermostat and Furnace Control
Armstrong Air communicating furnace thermostat or control displays E0; system may not respond to commands.
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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
Loose communication bus wiring, Incorrect thermostat wiring polarity, Faulty communicating thermostat, Faulty furnace control board
How to Fix / Troubleshooting
Safety: Turn off power to the furnace before working on thermostat or control wiring.
Steps to check:
- Inspect communication wiring: For Armstrong Air communicating systems, verify the A, B, C (or similar) terminals are wired consistently from thermostat to furnace and any outdoor unit. Tighten all terminal screws.
- Check polarity: Ensure the communication wires are not reversed between devices. Refer to the Armstrong Air wiring diagram on the furnace door.
- Power cycle: Turn off power to the furnace and thermostat for 2–3 minutes, then restore. This can reset communication.
- Bypass thermostat (if possible): If the system supports non-communicating mode, temporarily connect R to W at the furnace to see if the furnace will heat. If it does, the thermostat or communication module may be faulty.
- Replace faulty device: If wiring is correct and E0 persists, replace the communicating thermostat or furnace control board as indicated by further diagnostics.
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Repair Difficulty
Hard
4/5
Required Part
Communicating Thermostat
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