BoilerFaultCodes.com
AL21 High Lockout

Your boiler has shut down because a temperature sensor on the burner has detected an issue or has failed.

Call an engineer soon

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

Technical description: Burner thermostat fault

What causes this fault?

This fault occurs when a safety sensor detects that the burner area has become too hot, or if the sensor itself has developed an electrical fault. It is usually caused by a lack of water flow through the heat exchanger or a buildup of debris that prevents heat from being transferred away quickly enough. To prevent damage to internal components, the boiler automatically shuts down for safety.

Typical repair cost £120-£240 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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