High Lockout
F.78 on Vaillant ecoTEC

Your boiler is confused because it is trying to communicate with an external control box that isn't compatible with its current setup.

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

wrong configuration with accessory | link box VR65 connected to combination boiler

What causes this fault?

This fault occurs when the boiler's main control board detects an external wiring box that was designed for older hot water systems but is not compatible with your specific boiler model. It essentially creates a digital 'clash' where the boiler receives signals it doesn't understand, leading it to shut down for safety. The most common reason is that an incorrect accessory was installed or the boiler's internal software has been configured for a system type that doesn't match the physical parts attached.

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.70View →F.61View →F.62View →

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