F.10
Vaillant
Return Temperature Sensor Open Circuit
Boiler shows F.10, no heating or hot water
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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
Faulty return NTC sensor, Loose or broken return sensor wiring, Connector not seated on PCB, Failed PCB sensor channel
How to Fix / Troubleshooting
Safety: Isolate power and gas before removing covers. Allow the boiler to cool.
- Check system pressure and air: Ensure system pressure is 1.0–1.5 bar and bleed radiators to remove air. Air pockets can cause abnormal temperature readings.
- Locate return NTC: Find the return temperature sensor on the return pipe entering the boiler. Confirm it is firmly clipped to the pipe and the plug is fully inserted.
- Test continuity: With power off, disconnect the sensor and measure resistance. An infinite reading indicates an open circuit; replace the return NTC.
- Inspect harness: Follow the cable back to the PCB, checking for cuts, rodent damage, or pulled connectors. Reseat the connector on the PCB.
- Engineer check: If the sensor and wiring are OK but F.10 persists, a PCB fault is likely and should be confirmed and replaced by a professional.
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Repair Difficulty
Medium
3/5
Required Part
Return NTC Temperature Sensor
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