BoilerFaultCodesYour boiler has shut down because the internal gases have reached a temperature that is dangerously high for the system.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: FLUE GAS OVERHEAT
What causes this fault?
This fault occurs when the gases produced by the burner become too hot, triggering a safety sensor to prevent damage to the boiler or flue. The most common reasons are a build-up of debris or limescale inside the main heat exchanger or a failing pump that isn't moving water through the system fast enough to carry the heat away.
This issue is more frequent in winter when the boiler is working under maximum load for long periods, which puts additional strain on a struggling heat exchanger or pump.
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.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like L9 — no call-out fees, no surprises.
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