AF Emergency

Unical AF Fault Code

The Unical AF fault code means: your boiler has detected that dangerous exhaust fumes are not escaping through the flue correctly and has shut down to prevent them from entering your home.. It appears on 2 Unical models in our database.

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description: Flue gas anti-overflow thermostat
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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Unical models that show the AF fault code

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

Source

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

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