High Lockout
F.78 on Vaillant ecoTEC Plus 635

Your boiler is having trouble communicating with an external hot water control component, meaning it doesn't know the temperature of the water heading to your taps.

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

Interruption to DHW outlet sensor at external control. UK link box is connected, but domestic hot water NTC not bridged

What causes this fault?

This fault occurs when the boiler's main control board loses communication with the external sensors that monitor your hot water temperature. It is usually caused by a loose wiring connection between the boiler and the external control box or a sensor that has failed and is no longer sending a signal. Essentially, the boiler has entered a fail-safe mode because it cannot verify how hot the water is becoming.

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.13View →F.77View →

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