BoilerFaultCodesYour boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Control / no flow thermistor fault
What causes this fault?
This fault occurs when the boiler detects a sudden, unexpected rise in temperature because water isn't moving through the heat exchanger quickly enough. The most common reasons are a failed internal pump, a physical blockage like sludge in the pipework, or a faulty sensor that is sending incorrect temperature readings to the main computer.
This issue often appears when the heating is first turned on after summer, as pump components 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 L4 — 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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