AL21 High

Pro AL21 Fault Code

The Pro AL21 fault code means: your boiler has detected an issue with a safety device or an external thermometer, causing it to shut down to prevent overheating.. 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: External device fault (UHT/CPA)
Typical repair cost £90-180 (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 AL21 fault code

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

Source

Pro AL21 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