How much does it cost to fix boiler error code On On Flash?

Typical repair cost for the boiler On On Flash fault is £250-400 including labour.

On On Flash High

Boiler Fault Code On On Flash

Also written as ONONFLASH — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 31 August 2026

Fault code On On Flash appears across 3 boiler brands and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler's internal electronic brain has detected a critical error and is unable to safely control the heating system, meaning the boiler has shut down. (As reported on Potterton Promax 15/2 HE Plus)

Pick your boiler brand for the On On Flash fix

Baxi On On Flash 2 models Main On On Flash 1 model Potterton On On Flash 2 models

Source

Information for the On On Flash fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026