BoilerFaultCodesIdeal 424 Fault Code
The Ideal 424 fault code means: your boiler has detected that the internal temperature sensors have been wired or installed in the wrong positions, preventing it from heating safely.. It appears on 2 Ideal models in our database.
Technical description: Flow and return sensors NTC1 and NTC2 reversed
What causes the Ideal 424 fault?
This fault occurs when the boiler's control board detects that the water temperature readings are backwards, usually following a recent repair or component replacement. It is physically caused by the flow and return temperature sensors being plugged into the wrong ports on the main circuit board or clipped onto the incorrect internal pipes. Because the boiler cannot accurately track how heat is moving through the system, it shuts down to prevent overheating.
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 424 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal 424 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