BoilerFaultCodesYour boiler has detected that the water coming back from your radiators is hotter than the water going out, which usually means there is a blockage or the internal sensors are confused.
This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.
Technical description: Return temperature is higher than flow temperature. Burner operates itself after a small amount of time but if the problem repeats you need to Contact a heating engineer
What causes this fault?
This fault occurs when the boiler's internal sensors detect that the water returning from your radiators is hotter than the water it just sent out, which is physically impossible under normal operation. The most common reason is a blockage in the pipework or a failing pump preventing water from moving correctly, though it is often simply caused by one of the internal temperature sensors becoming faulty or sending incorrect data to the control board.
This issue is more frequent in autumn when heating systems are first turned on after a long summer break, as internal pump parts are more likely to seize or become stuck.
DIY fix possible
Check that all radiator valves (TRVs) are fully openEnsure the boiler pressure is between 1.0 and 1.5 barReset the boiler to see if the temporary sensor error clears
Find a Gas Safe engineer
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A boiler cover plan would cover faults like J6-21 — no call-out fees, no surprises.
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