High Lockout
F.01 on Vaillant Thermocompact

Your boiler has stopped working because one of its internal temperature sensors is unable to send a signal to the main control board.

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 | Return-NTC cable defective/broken NTC faulty

What causes this fault?

This fault occurs when the boiler's return sensor fails to communicate the water temperature back to the main control board. The most common reason is that the sensor's internal electrical resistance has failed due to age, or the thin wiring connecting it to the 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.

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Related fault codes

F.00View →F.10View →F.11View →

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