High Lockout
-9OC on Glow-worm Ultracom 2 CXI

Your boiler has detected a problem with the sensor that monitors hot water temperature, likely due to a loose connection or a fault in the internal wiring.

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

Check that the DHW thermistor wires are unconnected and are not faulty or trapped.

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor responsible for monitoring your hot water temperature. The most common reason is a loose electrical connection or a break in the internal wiring, preventing the system from knowing if the water is at a safe temperature. Because the boiler cannot verify the heat level, it stops providing hot water as a safety precaution.

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

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