S.00 Low

Vaillant S.00 Fault Code

Also written as S00 — same fault.

The Vaillant S.00 fault code means: your boiler is working correctly but is currently idle because your thermostat or programmer is not telling it to turn on the heating.. It appears on 21 Vaillant models in our database.

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: Heating mode error – no heat demand

What causes the Vaillant S.00 fault?

This status occurs when the boiler is waiting for a signal from your thermostat or timer to begin heating the radiators. The most common reason is that the room temperature is already higher than the thermostat setting or the programmer is currently in an 'off' period. It essentially means the boiler is sitting in standby mode because it believes no heat is currently required in your home.

This is frequently seen during the transition into Spring or Autumn when warmer outdoor temperatures mean the indoor thermostat is satisfied and doesn't trigger the boiler.

Typical repair cost £0-80 (including labour)

DIY fix possible

Check that your room thermostat is set higher than the current room temperatureEnsure your heating programmer or timer is set to 'ON' or 'AUTO'Replace the batteries in your wireless thermostat if the screen is blank

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Vaillant models that show the S.00 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant S.00 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026