BoilerFaultCodesYour boiler has detected that the temperature sensors monitoring the water going out and coming back are swapped or wired incorrectly, preventing it from heating safely.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow and return sensors NTC 1 and NTC 2 reversed
What causes this fault?
This fault occurs when the boiler detects that the pipe meant to be getting hot is staying cool, while the return pipe is heating up instead. It is usually caused by the temperature sensors being clipped onto the wrong pipes during a repair or a fault in the internal wiring harness that confuses the control board.
This fault is most common immediately after summer maintenance or a parts replacement, as it usually stems from human error during reassembly.
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 424 — no call-out fees, no surprises.
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