High Lockout
F.11 on Vaillant ecoTEC Pro / ecoTEC Plus

Your boiler has stopped working because a sensor that monitors the temperature of the water returning to the unit has developed an electrical fault.

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 NTC short circuit

What causes this fault?

This fault occurs when the sensor that monitors cooled water returning to the boiler develops an internal electrical failure. The most common reason is that the sensor has suffered an electrical short circuit, often due to moisture ingress or general wear and tear over time. When this happens, the boiler can no longer safely calculate the temperature difference across the heat exchanger, so 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.10View →

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