Boiler does not ignite, display shows E01 shortly after a heating or DHW demand.

Safety first: Turn off the boiler using the main power switch and close the gas isolation valve before removing any covers. Only a qualified gas engineer should work on gas-carrying components and combustion chambers.

• Reset the boiler: Press the reset button according to the Thermona user manual and observe if the boiler attempts to ignite. Listen for the fan, click of the gas valve, and ignition spark.
• Check gas supply: Ensure the gas isolation valve under the boiler is fully open. Confirm other gas appliances in the property work correctly. If not, contact your gas supplier.
• Inspect flue terminal: From outside, make sure the Thermona concentric flue terminal is not blocked by debris, snow, or vegetation, which can affect combustion and ignition.
• Electrode inspection (engineer level): Remove the front casing and combustion chamber cover. Check the ignition and ionisation electrodes for cracks, heavy soot, or incorrect gap. Clean gently with fine emery paper and refit, ensuring correct position relative to the burner.
• Burner and gas valve check (engineer level): Inspect the Thermona burner for heavy contamination or corrosion. Check inlet gas pressure and burner pressure with a manometer at the Thermona gas valve test points and adjust according to the service manual.
• PCB and wiring: Verify the electrode leads are firmly connected to the main PCB and not damaged. If ignition sequence is not initiated at all, the main PCB or ignition module may be faulty and require replacement.

If E01 persists after basic checks, stop attempting resets and contact a qualified Thermona service technician.