Low
S.2 on Vaillant ecoMAX 635

Your boiler is preparing to heat your home and is checking that water is moving through the pipes before it lights 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

Pump running – pre ignition (heating operation)

From this model's manual
Verified against the Vaillant Ecomax 613 installation & service manual, page 45.
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 when the boiler's internal computer tells the pump to start moving water around your heating system in preparation for ignition. It is a normal part of the startup sequence where the boiler confirms there is sufficient water flow before it safely fires the burner. If the code stays on screen for a long time without the boiler firing, it may indicate that the pump is struggling to move water or the pressure sensor is having difficulty detecting the flow.

DIY fix possible

Check that your external heating controls or thermostat are set to 'On' and a high temperatureEnsure all radiator valves are open to allow water flowWait up to 10 minutes, as this is often a normal part of the boiler's start-up sequence

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