BoilerFaultCodesYour boiler has detected that it is getting too hot and has shut down as a safety precaution to prevent damage.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: ALTERNATING 'H9' GO TO FRAME 93
What causes this fault?
This fault occurs when the water temperature inside your boiler rises too quickly or exceeds a safe limit, causing a safety sensor to trip. The most common reasons are a failed pump that isn't circulating the water properly or a blockage in the pipework that is restricting flow. In some cases, it can also be triggered by air trapped inside the system or a build-up of limescale in the heat exchanger.
This fault is more common in autumn when heating systems are switched back on after being idle all summer, which can cause internal pump parts to seize.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A boiler cover plan would cover faults like H9 — no call-out fees, no surprises.
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