High Lockout
F.64 on Vaillant ecoMAX

Your boiler has detected a communication error between its internal sensors and the main control board, preventing it from heating your home safely.

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

Electronics/NTC fault

What causes this fault?

This fault occurs when the boiler's main computer receives conflicting or impossible signals from its internal temperature sensors. It is usually caused by a short circuit in the wiring or a hardware failure within the control board itself, which prevents the system from accurately monitoring the water temperature. Because the boiler cannot guarantee it is operating at a safe heat level, 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.61View →F.62View →F.63View →F.65View →F.67View →

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