BoilerFaultCodesDaikin 12-66 Fault Code
Also written as 1266 — same fault.
The Daikin 12-66 fault code means: your boiler is having trouble detecting the steady flame it needs to run safely, but it may just be a temporary sensing error.. It appears on 1 Daikin model in our database.
Technical description: Ion base value lower than the upper factory limit, try resetting the boiler
What causes the Daikin 12-66 fault?
This fault occurs when the boiler's internal sensor fails to detect a strong enough electrical signal from the flame during operation. It is often caused by a dirty or worn-out sensing electrode, though it can also happen if the burner isn't receiving the correct mix of gas and air. To protect your safety, the boiler stops when it cannot verify that the gas is burning correctly.
DIY fix possible
Locate the reset button (usually a circular arrow or 'Reset' text) on the control panel.Press and hold the reset button for five seconds.Wait for the boiler to restart and monitor if the code clears.
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Gas Safe Register →Daikin models that show the 12-66 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Daikin 12-66 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