BoilerFaultCodesIdeal H4 Fault Code
The Ideal H4 fault code means: your boiler has detected an issue with the sensor that monitors the temperature of the exhaust gases, causing the system to shut down as a safety precaution.. It appears on 4 Ideal models in our database.
Technical description: FLUE THERMISTOR ERROR
What causes the Ideal H4 fault?
This fault occurs when the control board loses communication with the sensor that monitors the temperature of the gases leaving your chimney. The most common reason is that the sensor itself has developed an electrical fault or the wiring connecting it to the main board has become damaged or loose. Because this sensor is a critical safety component designed to prevent the boiler from overheating, the system will shut down immediately to protect the appliance.
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 H4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal H4 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