AL20 High

Pro AL20 Fault Code

The Pro AL20 fault code means: your boiler has switched itself off because it has detected that an internal component is overheating or getting too hot to operate safely.. It appears on 1 Pro model in our database.

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: Limit thermostat fault/fumes thermostat fault
Typical repair cost £120-£250 (including labour)

Engineer required

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

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Pro models that show the AL20 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Pro AL20 fault code information on this page is sourced and cross-checked against the official Pro service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026