High Lockout
S.21 on Vaillant ecoTEC Pro / ecoTEC Plus

Your boiler is struggling to start the fan when you turn on your hot water taps, meaning the system cannot safely clear exhaust gases to begin heating.

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

DHW mode – fan start up error

What causes this fault?

This fault occurs when the boiler's main control board sends a signal to start the fan, but it fails to rotate or reach the required speed to safely vent exhaust gases. The most common reasons are a buildup of dirt causing the fan bearings to seize or an electrical failure within the fan's motor or the wiring connecting it to the boiler.

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

F.32View →S.31View →S.32View →

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