BoilerFaultCodesSaunier Duval E21 Fault Code
The Saunier Duval E21 fault code means: your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.. It appears on 4 Saunier Duval models in our database.
Technical description: Lack of water (low pressure)
DIY fix possible
Locate the filling loop (a silver flexi-hose) connected to the underside of the boiler.Open the small tap or valve on the filling loop to allow cold mains water into the heating system.Monitor the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close the valve tightly and ensure the boiler resets itself.
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Gas Safe Register →Saunier Duval models that show the E21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Saunier Duval E21 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