High Lockout
437 on Ideal Response SE

Your boiler has stopped working because a temperature sensor has lost its connection or has developed a fault.

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

NTC 2 open

What causes this fault?

This fault occurs when the boiler's control board loses communication with the temperature sensor that monitors water leaving the unit. The most common reason is that the sensor has physically failed internally or the electrical wiring connecting it to the main circuit board has become loose or damaged. Without this data, the boiler shuts down as a safety precaution because it cannot accurately monitor how hot the water is getting.

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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