High Lockout
H4 on Ideal Mexico HE

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

FLUE THERMISTOR ERROR

What causes this 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 is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

H1View →

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