Medium Lockout
F.66 on Vaillant Thermocompact

Your boiler's main control panel is having trouble communicating with the internal display screen, meaning you cannot see or adjust your heating settings.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Display fault | Display defect; fault in the connection to the display; electronics defect

What causes this fault?

This fault occurs when the boiler's 'brain' can no longer talk to the screen on the front of the unit. The most common reason is a loose or damaged data cable inside the boiler, or a component failure on one of the circuit boards caused by age and heat. Because these parts cannot share information, the boiler stops working to prevent you from running it without any visual control.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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