BoilerFaultCodesBaxi ON ON ON Fault Code
Also written as ONONON — same fault.
The Baxi ON ON ON fault code means: your boiler has detected an internal electrical communication problem between its main control components.. It appears on 1 Baxi model in our database.
Technical description: Low voltage wiring harness error
What causes the Baxi ON ON ON fault?
This fault occurs when the internal electrical signals between the boiler's control systems and the gas valve are interrupted or lost. The most common reason is a loose connection in the wiring or a degradation of the copper pins within the harness, which prevents the computer from safely controlling the boiler's operation.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A cover plan would handle faults like ON ON ON: no call-out fees, no surprise bills.
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Baxi models that show the ON ON ON fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Baxi ON ON 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