S.31 Low

Vaillant S.31 Fault Code

Also written as S31 — same fault.

The Vaillant S.31 fault code means: your boiler is not providing heating because the central heating control knob on the front panel has been turned to the 'off' position.. It appears on 34 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: Central heating thermostat knob turned off

What causes the Vaillant S.31 fault?

This code appears when the physical heating temperature dial on the front of the boiler has been turned all the way to the left, which electronically disables the heating function. It is often caused by someone accidentally bumping the dial while cleaning or intentionally turning it off during the summer and forgetting to turn it back up. Because the dial is in the 'off' position, the boiler is simply following instructions to ignore any requests for heat from your thermostat.

This is most frequently encountered during the first cold spell of autumn when homeowners try to turn their heating back on after leaving the dial off all summer.

DIY fix possible

Locate the central heating control knob (usually marked with a radiator symbol) on the front of the boiler.Turn the knob clockwise to the desired temperature setting.Check if the display changes and the boiler begins its ignition sequence.

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Vaillant models that show the S.31 fault code

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

Source

Vaillant S.31 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 27 April 2026