Low
S.40 on Vaillant Atmomax

Your boiler has detected a minor inconsistency in its temperature readings and has entered a restricted performance mode to keep providing some heat while protecting the internal components.

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.

What the manual says

Manufacturer manual

Comfort safety mode active

Shared across the range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Check that all radiator valves are fully openEnsure the central heating system pressure is between 1.0 and 1.5 barPress the reset button (flame symbol with a cross) to clear the status code

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