BoilerFaultCodesIdeal 21 Fault Code
The Ideal 21 fault code means: your boiler has shut down because a safety sensor has detected that the burner area is getting too hot or is failing to register a secure connection.. It appears on 1 Ideal model in our database.
Technical description: Burner thermostat is open or not connected; check the thermostat for the switching point 107˚C. Check the burner cover for defects, it is possible that the flame will extend to the rear-side of the burner cover. Check the sealing between the mixing chamber and the heat exchanger and check the connection of the burner housing to the heat exchanger.
What causes the Ideal 21 fault?
This fault occurs when a safety sensor detects that the area around the burner is overheating, usually because heat is escaping where it shouldn't. The most common reason is a perished seal or a gap between internal components, which allows hot combustion gases to leak out and trigger the safety shut-off. It can also be caused by the sensor itself failing or a loose wiring connection preventing the boiler from confirming it is safe to fire.
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 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal 21 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