AL28 High

Pro AL28 Fault Code

The Pro AL28 fault code means: your boiler has detected that the water coming back from your radiators is hotter than the water going out, which usually means the water isn't flowing through the system correctly.. 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: Temperature differential inverted (return sensor temperature higher than thermistors flowsensor temperature)
Typical repair cost £120-280 (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 AL28 fault code

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

Source

Pro AL28 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