BoilerFaultCodesYour boiler has detected that the water temperature has reached a dangerously high level, causing it to shut down 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 'H' '3' GO TO FRAME 92
What causes this fault?
This fault occurs when the water inside the heat exchanger or the pipework gets too hot, triggering a safety shut-off to protect the boiler. The most common reasons are a failed pump that isn't moving water through the system fast enough, or a blockage caused by a build-up of sludge and debris. It can also be caused by a sensor that has become faulty and is sending the wrong temperature reading to the main control board.
This fault is often seen at the start of winter when the heating is turned on for the first time after summer, as pumps can seize up during long periods of inactivity.
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 H3 — no call-out fees, no surprises.
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