Low
S.21 on Vaillant ecoTEC plus Regular

Your boiler is performing a routine initial check of its internal air fan before it attempts to ignite the burner.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

What the manual says

Manufacturer manual

Hotwaterhandlingmode:Advancefanoperation

From this model's manual
Verified against the Vaillant Ecotec Plus Regular installation & service manual, page 32.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This is a normal status message rather than a breakdown, occurring every time the boiler prepares to heat your water or radiators. It happens when the control board sends power to the internal fan to clear out any residual gases and ensure a safe airflow before the burner lights. If the boiler stays on this code without firing, it usually means the fan is unable to reach the required speed or the air pressure sensor isn't detecting the flow.

DIY fix possible

No action is required as this is a normal status message, not a fault code.Wait a few minutes to see if the boiler moves on to the next stage of its operation.If the code stays on screen for a long time without the boiler firing, try resetting the boiler once.

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