ON FLASH ON High

Baxi ON FLASH ON Fault Code

Also written as ONFLASHON — same fault.

The Baxi ON FLASH ON fault code means: your boiler has detected a problem with its internal temperature sensor and has stopped working as a safety precaution.. 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: Thermistor error

What causes the Baxi ON FLASH ON fault?

This fault occurs when the boiler's internal temperature sensor, known as a thermistor, fails to send a reliable signal to the main control board. The most common reason is that the sensor has developed an electrical fault or has simply worn out over time, meaning it can no longer accurately monitor the water temperature. As a result, the boiler shuts down to prevent itself from overheating.

Typical repair cost £100-£180 (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 FLASH ON fault code

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

Source

Baxi ON FLASH ON 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