F3
Armstrong Air
Evaporator Coil Sensor Fault / Freeze Protection
Indoor unit flashes F3, blower may shut down during cooling, coil may be very cold
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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
Failed evaporator thermistor, Sensor not attached firmly to coil, Low airflow across coil, Low refrigerant charge
How to Fix / Troubleshooting
Safety first: Turn off power to the indoor unit. Coil fins are sharp—use gloves.
Steps:
- 1. Check filter and airflow: Replace dirty filters and ensure returns and supplies are not blocked.
- 2. Inspect coil sensor: Locate the thermistor clipped to the evaporator coil tubing. Ensure it is firmly attached and not corroded.
- 3. Examine wiring: Follow the sensor wires back to the board. Repair any damaged insulation and reseat connectors.
- 4. Thaw coil if iced: If the coil is frozen, turn system OFF and run FAN ONLY until all ice melts.
- 5. Test sensor: With power off, measure sensor resistance and compare to the temperature chart. Replace if out of spec.
- 6. If F3 persists: The board’s sensor circuit may be faulty, or there may be a refrigerant issue requiring a technician.
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Repair Difficulty
Medium
3/5
Required Part
Evaporator Coil Temperature Sensor
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