Low
S.4 on Vaillant ecoTEC 65kW

Your boiler is currently working correctly and is actively running its heating cycle to warm your home.

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

Burnermode:Heating

From this model's manual
Verified against the Vaillant Ecotec 65kw installation & service manual, page 34.
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 indicating that your boiler is currently in 'Heating Mode' and the burner is successfully lit to provide warmth to your radiators. It appears when the internal thermostat calls for heat and the startup sequence has completed without any issues. Because this is a standard operating state rather than a failure, no repair is necessary.

This code is most frequently seen during the autumn and winter months when the central heating is programmed to be active for longer periods.

Engineer required

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

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