High Lockout
F.1 on Vaillant ecoMAX System 612

Your boiler has stopped working because it cannot correctly measure the temperature of the water returning to the unit due to a faulty sensor or connection.

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: NTC broken, NTC cable broken, Defective connection at NTC, Defective connection at electronics | Cause: Return–NTC cable defective/broken NTC faulty

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor itself has failed due to age or electrical failure, though it can also be caused by a loose connection or damaged wiring. Because the boiler can no longer safely monitor heat levels, it shuts down as a protective measure.

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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