High Lockout
1C on Ideal Maximiser SE

Your boiler has detected that a safety sensor meant to monitor exhaust gases is disconnected or has failed, causing the system to shut down for protection.

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

Connection for flue gas thermostat not closed.

What causes this fault?

This fault occurs when the boiler's control system loses its electrical connection to the sensor that monitors the temperature of the exhaust gases. The most common reason is a vibration-loosened wiring connector or a physical failure of the thermostat itself, which serves as a vital safety barrier to prevent the unit from overheating. Because the boiler can no longer verify that exhaust temperatures are safe, it will shut down immediately to prevent damage.

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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