High Lockout
F.64 on Vaillant ecoMAX 635

Your boiler has detected an electrical issue with its internal heat sensors or 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

Electronic system/sensor fault | Cause: Short circuit in flow or backflow NTC, electronic fault

What causes this fault?

This fault occurs when the boiler's internal computer receives conflicting or impossible signals from its temperature sensors. It usually happens because a sensor has suffered an electrical short circuit or the main control board has developed a hardware malfunction, making it impossible for the system to monitor heat levels safely.

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.01View →F.61View →F.62View →

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