High Lockout
F.1 on Vaillant ecoTEC

Your boiler has stopped working because one of its internal temperature sensors has lost connection or failed.

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 open circuit

What causes this fault?

This fault occurs when the boiler's control board can no longer detect a signal from the sensor that monitors water returning from your radiators. This is usually caused by a failure of the internal thermistor component or a loose electrical connection between the sensor and the main circuit board. Without this data, the boiler cannot safely regulate its temperature, so it shuts down as a 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.0View →F.10View →F.11View →

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