F4
Armstrong Air
High Temperature Limit / Supply Air Overheat in Cooling
Indoor unit flashes F4, blower may run but cooling is disabled, thermostat may show system fault
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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
Reversed refrigerant lines or mode wiring, Stuck reversing valve on heat pump, Indoor blower running too slow, Faulty supply air temperature sensor
How to Fix / Troubleshooting
Safety first: Turn off power to both indoor and outdoor units. Refrigerant piping can be hot or cold—avoid contact.
Steps:
- 1. Confirm system type: Verify whether the system is straight cool or a heat pump. Ensure thermostat is correctly configured.
- 2. Check reversing valve wiring (heat pump): Ensure the O/B wire is correctly connected at thermostat, indoor board, and outdoor unit per Armstrong Air diagram.
- 3. Inspect supply air sensor: Locate the sensor in the supply plenum. Ensure it is not touching the heat exchanger or other hot surfaces.
- 4. Measure sensor resistance: With power off, test the sensor and compare to the temperature chart. Replace if faulty.
- 5. Verify blower speed tap: Confirm the blower is set to the correct cooling speed on the indoor board (refer to the airflow chart on the unit).
- 6. If F4 continues: A miswired or stuck reversing valve or control board fault may be present—contact a technician.
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Repair Difficulty
Hard
4/5
Required Part
Supply Air Temperature Sensor
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