High Lockout
b5 on Viessmann Vitodens 333

Your boiler's main internal computer has encountered a communication error and cannot control the heating process correctly.

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 - Internal error

What causes this fault?

This fault occurs when the boiler's central brain suffers a hardware failure or a critical data mismatch within its internal circuits. The most common reason is a breakdown of the communication between the main circuit board and the specialized coding plug that stores the boiler's specific operating instructions.

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

b1View →b7View →

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