High Lockout
b1 on Viessmann Vitodens 333

Your boiler's main control panel is having trouble talking to the internal computer, meaning you cannot adjust settings or operate the unit effectively.

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

Control mode - Communication fault – programming unit (internal)

What causes this fault?

This fault occurs when the data link between the boiler's main display screen and its internal control board is interrupted. The most common reason is a loose internal connection cable or a electronic component failure within the programming unit itself. Because these two parts can no longer 'talk' to each other, the boiler cannot receive commands to provide heating or hot water.

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

b0View →b7View →b8View →

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