BoilerFaultCodesTechnical description
Flue gas thermostat active: check flue gas outlet/air inlet.
What causes this fault?
This fault occurs when the internal safety sensor detects that the exhaust gases leaving the boiler are reaching dangerously high temperatures. The most common cause is a significant buildup of debris or limescale inside the heat exchanger, which prevents heat from transferring to your water and instead sends it out through the flue. It can also be triggered if the flue pipe itself is blocked or if the internal seals have perished.
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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