BoilerFaultCodesVaillant S.40 Fault Code
Also written as S40 — same fault.
The Vaillant S.40 fault code means: your boiler is temporarily running at a lower power level to prevent a fault and ensure it can still provide some warmth while it detects a minor issue.. It appears on 8 Vaillant models in our database.
Technical description: Comfort protection active. The comfort protection mode is activated.
What causes the Vaillant S.40 fault?
This happens when the boiler senses that its internal temperatures are rising too quickly or are inconsistent, often due to a slight buildup of debris or a sensor that is beginning to fail. Instead of shutting down completely, the boiler reduces its heat output to protect itself from damage while still trying to keep your home warm. It is essentially the boiler's way of 'limping' along until the underlying circulation or sensing issue is fixed.
DIY fix possible
Check for any other error codes on the displayMonitor the boiler to see if it returns to normal operation automaticallyEnsure all radiator valves are open to allow for maximum heat dissipationIf the message persists, book a routine service with a Gas Safe engineer to check sensor readings
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the S.40 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant S.40 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