ON FLASH FLASH High

Main ON FLASH FLASH Fault Code

Also written as ONFLASHFLASH — same fault.

The Main ON FLASH FLASH fault code means: your boiler has detected that water is not moving through the system correctly or there isn't enough water inside the unit to operate safely.. It appears on 1 Main model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Circulation error or dry fire
Typical repair cost £120-350 (including labour)

DIY fix possible

Check the pressure gauge on the front of the boiler; if it is below 1.0 bar, use the filling loop to top it up to 1.5 bar.Ensure all radiator valves and isolation valves under the boiler are fully open.Gently bleed your radiators to remove any trapped air that might be blocking water flow.

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Main models that show the ON FLASH FLASH fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Main ON FLASH FLASH fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026