A5
Viessmann
Indoor Heat Exchanger Freeze / Frost Protection
Indoor unit stops in heating mode; A5 error shown; outdoor unit may still run briefly
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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
Low refrigerant charge, Severely restricted indoor airflow, Faulty indoor coil temperature sensor, Expansion device malfunction (electronic expansion valve)
How to Fix / Troubleshooting
Safety: Isolate power before opening the unit. Refrigerant work must be done by a certified technician.
- Clean filters and coil: Remove filters and clean. Inspect the indoor coil for heavy dust buildup; if dirty, gently clean with a soft brush and coil cleaner.
- Check fan speed: Run the unit and verify the indoor fan reaches full speed. If airflow is weak, investigate the fan motor and capacitor.
- Inspect coil sensor: Ensure the coil sensor is properly clipped to the coil and wiring is intact. Replace if damaged.
- Look for icing: During operation, check if the indoor coil is icing over. If so, turn the system off and allow it to thaw completely.
- Call technician for refrigerant/expansion valve: If A5 recurs with clean filters and good airflow, a technician should check refrigerant pressures and the electronic expansion valve operation.
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Repair Difficulty
Hard
4/5
Required Part
Electronic Expansion Valve
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