BoilerFaultCodesYour boiler has detected a fault with a safety sensor designed to prevent the main heating unit from overheating, causing the system to shut down.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: The safety cut-out in the primary heat exchanger is defective
What causes this fault?
This fault occurs when the boiler's internal safety switch detects that the main heat exchanger is getting too hot, or if the sensor itself has failed and is sending incorrect data. The most common reasons include a lack of water flow caused by a trapped air bubble, a failing pump, or a build-up of sludge that prevents heat from being transferred away from the unit quickly enough.
This fault is more common in autumn when heating systems are first turned on after a long summer break, as internal pumps can seize up or air can become trapped in the pipes.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A boiler cover plan would cover faults like F.78 — no call-out fees, no surprises.
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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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