BoilerFaultCodesDaikin J6-20 Fault Code
Also written as J620 — same fault.
The Daikin J6-20 fault code means: your boiler has detected that the water returning to the unit is too hot, which has caused it to shut down to prevent damage.. It appears on 1 Daikin model in our database.
Technical description: Return temperature sensor overheating due to a blocking fault or a lockout error – try resetting the boiler
What causes the Daikin J6-20 fault?
This fault occurs when the water traveling back into the boiler from your radiators is at a higher temperature than the system expects, which usually signals a circulation problem. The most common reasons are a pump failure that isn't moving water fast enough, or a blockage caused by internal debris or air trapped in the system. As a safety measure, the boiler shuts down to prevent the internal components from being damaged by this excessive heat.
This issue is more common during the first cold spell of autumn, as circulation pumps can seize up after being inactive over the summer months.
DIY fix possible
Ensure all radiator valves are fully open to allow water to circulate.Check that the central heating system pressure is between 1.0 and 1.5 bar.Press the Reset button on the boiler control panel to clear the lockout.
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Gas Safe Register →Daikin models that show the J6-20 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Daikin J6-20 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