BoilerFaultCodesYour boiler has detected that the temperature sensors for the water going in and out are reporting incorrect or swapped readings, which has caused the system 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: NEGATIVE DIFFERENTIAL FLOW OR RETURN THERMISTOR FAULT
What causes this fault?
This fault occurs when the boiler detects that the water pipe leaving the unit is cooler than the pipe returning to it, which is physically impossible during normal operation. The most common reason is that the internal temperature sensors have failed or their wiring has become cross-connected, leading the boiler to shut down to prevent overheating. It can also be triggered by a complete lack of water circulation caused by a seized pump or a significant blockage in the system.
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 FA — 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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