F9
Vaillant
Indoor Return Air / Auxiliary Temperature Sensor Fault
F9 error, unit stops and may show abnormal temperature readings
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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
Auxiliary NTC sensor open or shorted, Sensor dislodged from return air path, Damaged sensor harness, Indoor PCB sensor channel failure
How to Fix / Troubleshooting
Safety: Switch off power at the breaker before opening the indoor unit.
- Locate auxiliary sensor: Some Vaillant models have a second room or return air sensor near the filter or behind the front grille. Identify the small NTC sensor and its wiring.
- Check placement: Ensure the sensor is positioned in the airflow, not touching metal parts that could skew readings.
- Inspect wiring: Follow the cable to the PCB, checking for damage or loose connectors. Reseat the plug.
- Measure resistance: Unplug and test with a multimeter. Compare to the expected resistance at room temperature. Replace if out of range.
- Replace sensor: Install a new indoor auxiliary NTC sensor if needed, securing it in the correct location.
- Test: Restore power and verify that F9 clears and that room temperature readings are stable.
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Repair Difficulty
Medium
3/5
Required Part
Indoor Auxiliary NTC Temperature Sensor
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