High Lockout
F10 on Saunier Duval Isotwin F30E

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, which has caused the heating to stop working.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Central heating return thermistor fault. Thermistor cable defective/broken, thermistor faulty. Check that thermistor attached correctly to pipe

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that measures the temperature of the water coming back from your radiators. The most common reason is that the internal components of the sensor have failed due to age, or the electrical wiring connecting it to the main computer has become loose or damaged.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

F1View →F11View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →