F5
Saunier Duval Return Temperature Sensor Fault (NTC) / Overheat Detection

Boiler shows F5 and locks out, often associated with 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

Faulty return NTC sensor, Loose NTC connection, Blocked primary circuit or pump failure causing abnormal temperature differential

How to Fix / Troubleshooting

Safety first: Turn off power and let the boiler cool. Hot water in the system can cause burns.

Steps to check:

  • Access the return NTC sensor on the return pipe to the main heat exchanger.
  • Check the wiring harness and connector for damage or corrosion. Reseat firmly.
  • Test the NTC resistance with a multimeter and compare to the manufacturer's chart.
  • If the sensor is defective, replace it with a genuine Saunier Duval NTC.
  • Check that the system pump is running and that radiators are heating evenly; cold radiators may indicate circulation issues contributing to F5.

Warning: If you suspect a circulation blockage or pump failure, a professional power flush or pump replacement may be required.

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Repair Difficulty

Medium 3/5

Required Part

Return NTC Sensor
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