Boiler does not ignite, burner does not start, display shows E01 shortly after call for heat or hot water.

Safety first: Turn off the boiler using the main power switch and close the gas isolation valve. Wait at least 10 minutes for any unburned gas to dissipate. Do not attempt repair if you smell strong gas – contact a licensed gas technician immediately.

• Check gas supply: Verify that the gas isolation valve to the Defro boiler is fully open and that other gas appliances in the home work. If there is no gas supply, contact your gas provider.
• Reset the boiler: Use the front control panel reset button (refer to your Defro model manual). If E01 clears and the boiler ignites normally, monitor for recurrence.
• Inspect ignition electrode: With power and gas OFF, remove the front cover. Locate the ignition electrode on the burner assembly. Check for cracked ceramic, loose cable, or heavy soot. Lightly clean the tip with fine emery cloth and ensure the cable is firmly connected to the ignition transformer on the Defro control board.
• Check flame sensor (ionization electrode): Clean the electrode with a non-abrasive pad and ensure it is correctly positioned in the flame path. Confirm the grounding wire from the burner to the boiler chassis is tight and corrosion-free.
• Burner inspection: Remove the burner from the Defro combustion chamber if accessible. Clean any heavy soot or debris from burner ports using a soft brush and vacuum. Reassemble carefully, ensuring all gaskets are seated.
• Call a professional: If E01 persists, a certified technician should test the Defro gas valve coil resistance, ignition transformer output, and combustion settings with a flue gas analyzer.