High Lockout
b4 on Viessmann Vitodens 333

Your boiler's internal computer is having trouble processing electrical signals correctly, which is preventing it from running its safety and efficiency tests.

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

Emissions test mode - Internal error analog converter

What causes this fault?

This fault occurs when the boiler's main brain fails to translate physical electrical signals into data it can understand. It is usually caused by an internal hardware failure within the circuit board's circuitry, meaning the boiler can no longer verify if it is burning fuel safely or efficiently. As a result, the system shuts down to prevent any unpredictable behavior.

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 →b8View →b7View →

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