BoilerFaultCodesSaunier Duval 21 Fault Code
The Saunier Duval 21 fault code means: your boiler has stopped working because the water pressure inside the heating system has dropped too low.. It appears on 2 Saunier Duval models in our database.
Technical description: Water pressure too low: open the blue tap (t) situated under the boiler until a pressure of 1.5 bar is obtained.
DIY fix possible
Locate the blue filling loop tap found underneath the boiler unit.Slowly turn the blue tap anti-clockwise to let water enter the system.Watch the pressure gauge on the front of the boiler until it reaches 1.5 bar, then turn the tap firmly off.
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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 31 August 2026