High Lockout
F.11 on Vaillant ecoMAX

Your boiler has detected an electrical fault with the sensor that monitors water returning from your radiators, causing the system to shut down for safety.

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

Return temperature sensor short-circuit

What causes this fault?

This fault occurs when the electrical sensor that monitors water returning to the boiler develops an internal short-circuit. It is often caused by heat degradation over time or water damage to the sensor's wiring, which leads to the control board receiving an incorrect electrical signal. Because the boiler can no longer accurately monitor temperature, it shuts down as a safety precaution.

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.

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Related fault codes

F.01View →F.10View →

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