BoilerFaultCodesYour boiler has detected that the temperature sensors are reading back-to-front or are failing, 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: NEGATIVE DIFFERENTIAL FLOW OR RETURN THERMISTOR FAULT
What causes this fault?
This fault occurs when the boiler's internal computer detects that the 'return' pipe is hotter than the 'flow' pipe, which is the opposite of how a heating system should function. The most common reason is that one of the temperature sensors has become faulty or unclipped, but it can also be triggered by a pump failure or a significant blockage that is pushing hot water back the wrong way.
This fault often appears when the heating is first turned on after summer, as pump components can seize during periods of inactivity.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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