BoilerFaultCodesVaillant lock out Fault Code
Also written as LOCKOUT — same fault.
The Vaillant lock out fault code means: your boiler has stopped working because it cannot detect a flame, most likely because the gas supply is interrupted or there is air trapped in the pipes.. It appears on 2 Vaillant models in our database.
Technical description: The 'lock out' indicator will illuminate. This usually means that the gas supply is turned off, or is not purged of air.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the lock out fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant lock out fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026