BoilerFaultCodesTechnical description
Flue gas detected inside boiler. Is the flue correctly installed and free of blockages?; Check air/gas channels in gas/water heat exchanger; Replace flue gas safety device.
What causes this fault?
This fault occurs when the boiler's internal sensors detect that poisonous exhaust gases are leaking back into the casing instead of being safely expelled outside. The most common reasons are a blockage in the flue pipe, a cracked internal seal, or heavy debris buildup within the heat exchanger that prevents the fumes from flowing away correctly.
This fault can be more frequent in autumn or winter if debris like fallen leaves or nesting birds have blocked the external flue terminal.
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.