BoilerFaultCodesYour boiler has detected that the water temperature sensors are giving conflicting readings, often caused by the flow and return pipes being switched or a sensor failing.
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 returning from your radiators is hotter than the water leaving the boiler, which is physically impossible under normal conditions. The most common reasons are that the flow and return pipes were crossed during installation, or one of the internal temperature sensors has developed a fault and is sending the wrong signal to the control board.
This fault often appears immediately after summer if homeowners have had new radiators or a new boiler installed while the heating was turned off.
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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