Medium Lockout
S.03 on Vaillant ecoTEC Pro / ecoTEC Plus

Your boiler is trying to start up to provide heating, but it keeps failing to light the flame correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Heating mode – ignition sequence error

What causes this fault?

This fault occurs when the boiler attempts to start the combustion process but fails to detect a stable flame after several attempts. It is usually caused by a worn-out ignition spark plug that can no longer bridge the gap, or a failing gas valve that isn't providing the correct amount of fuel into the burner.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F.28View →F.29View →S.04View →

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