F.10
Vaillant
Return Temperature Sensor Open Circuit
Display shows F.10; water heater will not fire when hot water is demanded
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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
Defective return NTC sensor, Loose or oxidised NTC connector, Broken wiring between NTC and PCB, Faulty main PCB sensor channel
How to Fix / Troubleshooting
Safety: Turn off power and gas before opening the appliance. Only competent persons should work inside the unit.
- Identify the return NTC: On combination-type Vaillant water heaters, the return NTC is mounted on the return pipe to the heat exchanger.
- Check connection: Ensure the NTC plug is firmly seated on both the sensor and the PCB. Unplug and replug to wipe contacts.
- Test the sensor: Disconnect the NTC and measure resistance with a multimeter. Compare to Vaillant’s NTC chart. An infinite reading indicates an open sensor; replace it.
- Check wiring continuity: With power off, test continuity from the NTC plug to the PCB connector. Repair or replace the harness if open.
- Reassemble and test: Fit a new NTC if needed, secure wiring away from hot surfaces, restore power and gas, and reset the fault. Run the heater and confirm normal operation.
- If F.10 remains: The main control board may be defective. Contact an authorised Vaillant service partner for PCB diagnostics and replacement.
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Repair Difficulty
Hard
4/5
Required Part
Return Temperature NTC Sensor
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