BoilerFaultCodesYour boiler has shut down to prevent the heat exchanger from overheating, often because the water isn't flowing through the system correctly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Exchanger protection intervention
What causes this fault?
This fault occurs when the boiler's internal sensors detect that the heat exchanger is getting too hot too quickly, which usually indicates that water isn't moving through the unit fast enough to carry the heat away. The most common reason is a failing internal pump or a build-up of sludge and debris that is restricting the flow of water through the system. In some cases, it can also be caused by air trapped in the heat exchanger or a faulty temperature sensor giving an incorrect reading.
This fault is more common during the first few cold spells of autumn or winter when the heating is turned on after a long summer break, as pumps can seize up during periods of inactivity.
DIY fix possible
Check that your home heating system pressure is between 1.0 and 1.5 barEnsure all radiator valves and isolation valves are fully openTry resetting the boiler using the 'R' button once the unit has cooled down
Find a Gas Safe engineer
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A boiler cover plan would cover faults like F43 — no call-out fees, no surprises.
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