F.10
Vaillant Return Temperature Sensor Open Circuit

Furnace displays F.10 and will not fire, sometimes after overheating or poor circulation.

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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 connector at return sensor, broken wiring between return NTC and PCB, faulty PCB input channel.

How to Fix / Troubleshooting

Safety: Switch off the appliance and disconnect it from the mains before removing covers.

Check the return NTC:

  • Locate the return NTC on the return pipe entering the main heat exchanger or hydraulic block.
  • Inspect for physical damage, corrosion, or signs of overheating.
  • Unplug the sensor and measure resistance with a multimeter. Compare to Vaillant's NTC resistance chart; an open circuit indicates a failure.
  • Check the connector and harness for loose pins or broken conductors. Reseat the plug firmly on the PCB.
  • Replace the return NTC if readings are out of range or intermittent when the cable is flexed.

Professional help: If the sensor and wiring are confirmed to be in good condition, the main control board may be defective and should be tested or replaced by a certified technician.

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Repair Difficulty

Medium 3/5

Required Part

Return NTC Temperature Sensor
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