Low Warning
S.40 on Vaillant ecoTEC Pro / ecoTEC Plus

Your boiler has detected a minor inconsistency in its temperature readings and has slightly reduced its power to ensure it continues to provide heating and hot water safely.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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

Comfort safety mode active

What causes this fault?

This occurs when there is a significant difference between the flow and return temperature sensors, suggesting a possible circulation issue. To prevent overheating, the boiler chooses to run at a lower power level rather than shutting down completely. The most common reasons are air trapped in the system or a slight imbalance in the sensors' readings.

DIY fix possible

Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 bar on the digital displayReset the boiler by pressing the flame symbol button for three seconds

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Related fault codes

F.20View →F.22View →F.24View →

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