Lights: On Flash On High

Baxi Lights: On Flash On Fault Code

Also written as LIGHTSONFLASHON — same fault.

The Baxi Lights: On Flash On fault code means: your boiler has detected a faulty temperature sensor, which means it cannot accurately measure how hot the water is and has stopped running 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 fault - Go to Thermistor section of the fault finding instructions.
Typical repair cost £90-160 (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 Lights: On Flash On fault code

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

Source

Baxi Lights: 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