BoilerFaultCodesYour boiler has switched its control method to the return pipe because it is detecting a potential issue with the main flow sensor.
This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.
Technical description: Heating flow return regulation
What causes this fault?
This fault occurs when the boiler detects a logical error or an inconsistent reading from the flow temperature sensor. To keep your heating running safely, the boiler automatically switches to monitoring the return pipe sensor while it ignores the potentially faulty flow data. The most common reason is a failing or 'drifting' sensor that is sending incorrect electrical signals back to the main control board.
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 D.17 — no call-out fees, no surprises.
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