High Lockout
F.64 on Vaillant ecoTEC Plus 840

Your boiler has detected an internal communication error between its electrical sensors and the main control board, causing it to shut down for safety.

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. Short-circuit in flow or return NTC or electronics defective

What causes this fault?

This fault occurs when the boiler's main computer receives conflicting or impossible signals from the internal temperature sensors. The most common reason is an electrical short-circuit or a component failure on the main circuit board, which prevents the boiler from safely monitoring the water temperature.

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.

Find a Gas Safe engineer
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Related fault codes

F.61View →F.62View →F.63View →F.67View →

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