Low Information
S.03 on Vaillant Thermocompact

Your boiler is currently going through its routine process of lighting the flame to start heating your water.

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.

Technical description

Ignition sequence (heating operation)

What causes this fault?

This status code appears when the boiler is actively trying to light the gas burner to provide heating or hot water. It represents a normal part of the startup cycle where the internal computer checks the safety sensors before opening the gas valve and creating a spark. If this code stays on screen for a long time without the boiler firing up, it usually suggests a delay in the ignition process.

DIY fix possible

No action required; this is a normal status message and not a fault.Wait for the boiler to complete the ignition sequence.Ensure your thermostat is calling for heat or hot water.

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