F9
Vaillant
Indoor Return Air / Auxiliary Temperature Sensor Fault
F9 error: The unit stops and may display abnormal temperature readings.
Advertisements
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.
Advertisements
Possible Causes
Auxiliary NTC sensor open or shorted, Sensor dislodged from the 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 located near the filter or behind the front grille. Identify the small NTC sensor and its wiring.
- Check placement: Ensure the sensor is positioned correctly in the airflow and is not touching any metal parts that could skew readings.
- Inspect wiring: Follow the cable to the PCB, checking for any damage or loose connectors. Reseat the plug firmly.
- Measure resistance: Unplug the sensor and test its resistance with a multimeter. Compare the reading to the expected resistance at room temperature. Replace the sensor if it is out of range.
- Replace sensor: Install a new indoor auxiliary NTC sensor if necessary, ensuring it is secured in the correct location.
- Test: Restore power and verify that the F9 error clears and that room temperature readings are stable.
Advertisements
Repair Difficulty
Medium
3/5
Required Part
Indoor Auxiliary NTC Temperature Sensor
Check Price on Amazon
We may earn a commission from links on this page.
Advertisements