BoilerFaultCodesYour boiler has automatically shut down because the internal temperature of the exhaust gases has reached an unsafe level.
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 thermostat has interrupted the safety loop
What causes this fault?
This fault occurs when the gases being pushed out of your chimney reach a dangerously high temperature, triggering a safety sensor to shut the system down. The most common reason is a buildup of debris or limescale inside the main heat exchanger, which prevents heat from being transferred to your water and instead sends it out through the flue. It can also be caused by a faulty sensor sending an incorrect reading to the boiler's computer.
This fault is more frequent during the winter months when the boiler is under heavy load and running for long periods to heat the home.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
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A boiler cover plan would cover faults like F.25 — no call-out fees, no surprises.
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