BoilerFaultCodesIdeal P21 Fault Code
The Ideal P21 fault code means: your boiler has stopped working because it cannot accurately tell if there is enough water pressure in the system to operate safely.. It appears on 1 Ideal model in our database.
Technical description: Low Pressure Sensor Error
What causes the Ideal P21 fault?
This fault occurs when the boiler's internal computer loses communication with the sensor that monitors water levels. The most common reason is an electrical failure within the sensor itself or a loose connection in the wiring, meaning the boiler shuts down as a safety precaution because it cannot verify if it is safe to fire up.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Ideal models that show the P21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal P21 fault code information on this page is sourced and cross-checked against the official Ideal service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026