Emergency
Red LED illuminated (TTB) on Potterton Kingfisher MF 40-100

Your boiler has detected potentially dangerous exhaust fumes backing up into your home due to a blocked or obstructed chimney or flue, and has safely shut itself off to prevent carbon monoxide poisoning.

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

Products Discharge Safety Device (TTB) - CFL Only: In the case of flue blockage this device will operate and the boiler will go to lockout. Repeated shutdown should be investigated.

From this model's manual
Verified against the Potterton Boilerkingfisher Mfrsl Cfl40 100 installation & service manual.
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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