BoilerFaultCodesBaxi ON FLASH FLASH Fault Code
Also written as ONFLASHFLASH — same fault.
The Baxi ON FLASH FLASH fault code means: your boiler has detected that water is not moving through the system properly or there isn't enough water inside the unit to safely heat up.. It appears on 3 Baxi models in our database.
Technical description: Circulation error or dry fire
What causes the Baxi ON FLASH FLASH fault?
This fault occurs when the boiler's internal sensors detect that heat isn't being carried away from the heat exchanger quickly enough, usually due to a lack of water flow. The most common reasons are a failed circulation pump that can no longer push water through the radiators, or a blockage such as a build-up of sludge or air in the pipework.
This issue is frequently seen in autumn when heating systems are turned back on after a long summer break, as pumps can often seize up during periods of inactivity.
DIY fix possible
Check that your external heating system pressure is between 1 and 1.5 barEnsure all radiator valves and isolation valves are fully openGently bleed your radiators to remove any trapped air pockets
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Baxi models that show the ON FLASH FLASH fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Baxi ON FLASH FLASH fault code information on this page is sourced and cross-checked against the official Baxi service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026