Emergency
C12 on Sime Format B Combi

Your boiler has detected an issue with the air supply or exhaust system, meaning it has shut down to keep your home safe.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Act immediately

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

What the manual says

Manufacturer manual

C12

From this model's manual
Verified against the Sime Format B Combi installation & service manual, page 14.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a emergency severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

C11View →C13View →

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