F5
Armstrong Air
Thermostat Communication Fault (Communicating Systems)
Indoor unit flashes F5, system may short-cycle or lock out, thermostat may show communication error
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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
Incorrect thermostat wiring, Loose communication bus connections, Incompatible thermostat, Faulty thermostat
How to Fix / Troubleshooting
Safety first: Turn off power to indoor and outdoor units before changing thermostat wiring.
Steps:
- 1. Verify thermostat model: Ensure the thermostat is an Armstrong Air-approved communicating model for your system.
- 2. Check wiring at thermostat: Remove the thermostat from its base and inspect the terminals. Confirm each conductor is landed on the correct terminal per the installation manual.
- 3. Inspect wiring at indoor board: Match the thermostat conductors to the same labeled terminals at the indoor control board.
- 4. Look for splices: If there are splices in the thermostat cable, ensure they are secure and not corroded.
- 5. Reset thermostat: Perform a factory reset or communication reset per the thermostat manual, then reconfigure equipment.
- 6. If F5 persists: Replace the thermostat with a known-good compatible model or have a technician test the communication bus and indoor board.
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Repair Difficulty
Medium
3/5
Required Part
Communicating Thermostat
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