Low
S.15 on Vaillant ecoTEC exclusive Combi

Your boiler is performing a self-check to ensure water is moving correctly and the fan is working before it attempts to ignite.

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

Fan and water pump return water

From this model's manual
Verified against the Vaillant Ecotec Exclusive Combi installation & service manual, page 41.
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 status occurs as the boiler completes a safety sequence to confirm that the internal pump is circulating water properly and the fan is operating at the correct speed. It is a normal part of the start-up process where the system verifies all conditions are safe before allowing the burner to ignite. If the code stays on screen too long, it may suggest a slight delay in the pump or fan reaching their required operating levels.

DIY fix possible

No action is required as this is a normal status message, not a permanent error.Wait 5-10 minutes for the boiler to complete its internal system check.Ensure your radiator valves are open to allow water to circulate properly during the check.

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