BoilerFaultCodesDaikin J6-21 Fault Code
Also written as J621 — same fault.
The Daikin J6-21 fault code means: your 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.. It appears on 1 Daikin model in our database.
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 the Daikin J6-21 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
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Gas Safe Register →Daikin models that show the J6-21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Daikin J6-21 fault code information on this page is sourced and cross-checked against the official Daikin service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026