Boiler locks out with E02, fan starts but burner does not light, sometimes accompanied by multiple failed ignition attempts.

Safety warning: Work on gas components must be done by a licensed gas technician. Do not open or adjust the gas valve yourself.

• Visual check: Turn off power. Remove the front panel and locate the gas valve on the Defro gas train. Check for obvious damage, burning, or loose connectors on the valve coil.
• Check gas isolation valves: Ensure all manual gas cocks to the boiler are fully open. If there is a separate gas filter on the Defro boiler inlet, verify that its service valves are open.
• Reset and observe: Restore power, reset the boiler, and listen: the fan should start, then you should hear a click from the gas valve. If there is no click, the valve may not be energizing.
• Do not attempt calibration: Gas valve pressure settings and combustion tuning require specialized tools. If E02 persists, contact an authorized Defro service center to test inlet pressure, valve operation, and combustion.