Boiler locks out with E02, burner stops shortly after ignition or during operation.

Safety warning: Isolate electrical power and close the gas valve before removing covers. Flame supervision issues must be handled by a competent gas engineer.

• Check gas supply stability: Confirm other gas appliances run without interruption. If there are supply issues, contact your gas provider.
• Ventilation and flue: Ensure the boiler has adequate ventilation and the Thermona flue system is not obstructed or damaged. Poor combustion can cause intermittent flame loss.
• Reset and observe: Reset the boiler and watch the ignition sequence. If the flame appears then cuts out after a few seconds, the ionisation signal may be weak.
• Electrode and burner cleaning (engineer level): Remove the combustion cover. Clean the burner surface and the ionisation electrode with a soft brush and fine abrasive. Check the electrode position and cable integrity.
• Check earth connection: Ensure the boiler has a solid earth connection; poor earthing can affect flame detection.
• PCB diagnosis: If electrodes and burner are in good condition and gas pressures are correct, the main PCB may not be reading the flame signal correctly and may need replacement.

Do not continue to reset the boiler repeatedly; arrange a Thermona-authorised technician visit.