ON ON FLASH High

Baxi ON ON FLASH Fault Code

Also written as ONONFLASH — same fault.

The Baxi ON ON FLASH fault code means: your boiler's internal computer has developed a fault and can no longer control the heating process safely.. It appears on 3 Baxi models 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

What causes the Baxi ON ON FLASH fault?

This fault occurs when the electrical components on the main control board suffer a critical failure, preventing the system from processing instructions correctly. The most common reason is general wear and tear over many years of use, though it can also be triggered by a sudden power surge or moisture getting into the casing.

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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Baxi models that show the ON ON FLASH fault code

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

Source

Baxi ON ON 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