BoilerFaultCodesSaunier Duval 21 Fault Code
The Saunier Duval 21 fault code means: your boiler has stopped working because the water pressure inside the system is currently too low to operate safely.. It appears on 4 Saunier Duval models in our database.
Technical description: You have insufficient water pressure (0.5 bar or lower).
DIY fix possible
Locate the filling loop (a flexible silver hose) underneath the boiler.Open the valves on the filling loop to allow cold mains water into the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close both valves firmly and ensure the filling loop is disconnected if it is a temporary hose.
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Gas Safe Register →Saunier Duval models that show the 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Saunier Duval 21 fault code information on this page is sourced and cross-checked against the official Saunier Duval service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026