ON ON FLASH High

Main ON ON FLASH Fault Code

Also written as ONONFLASH — same fault.

The Main ON ON FLASH fault code means: your boiler's internal computer has encountered a critical error and needs a professional electronic inspection.. 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: Printed circuit board (PCB) error
Typical repair cost £280-450 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Main models that show the ON ON FLASH fault code

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

Source

Main ON ON 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