High Lockout
b4 on Viessmann Vitodens

Your boiler has not been fully configured for operation or there is a communication problem between the internal controls and the burner assembly.

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

Installation in unfinished state - Fault in burner / Ignition valve

What causes this fault?

This fault occurs when the boiler's internal computer cannot successfully communicate with the burner system or hasn't been properly programmed to start. It is often caused by a loose connection, a faulty ignition lead, or a breakdown in the electronic control board that manages the gas flow. Effectively, the boiler has safety-locked because it cannot verify that it is safe to ignite the flame.

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 →F4View →

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