BoilerFaultCodesYour boiler has shut down because its internal safety sensor has detected that the system is getting dangerously hot.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: External temperature limiter has triggered (STB).
What causes this fault?
This fault occurs when the water inside the boiler reaches an unsafe temperature, causing the safety switch to trip and shut everything down to prevent damage. The most common reasons are a failed pump that isn't moving hot water away from the heat exchanger quickly enough, or a blockage within the system pipework.
This fault is more common when the heating is first turned on after summer, as circulation pumps can seize up after 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
Search the official register for a qualified engineer in your area.
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A boiler cover plan would cover faults like D3 — no call-out fees, no surprises.
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