Boiler locks out with E02, burner stops shortly after ignition or during operation, sometimes accompanied by rumbling or popping noises.

Safety: Switch off electrical power and close the gas valve before removing any covers. Do not attempt to adjust gas pressure yourself.

• Reset and observe: Reset the boiler and watch the ignition sequence through the inspection window (if present). If the flame lights then goes out and E02 appears, there is likely a combustion or flame sensing issue.
• Check ventilation and flue: Ensure air inlets and the flue terminal are not obstructed. Poor air supply can cause unstable flame.
• Check system pressure: Low water pressure can cause overheating and flame instability. Confirm the pressure gauge is in the green zone (typically 1.0–1.5 bar when cold). Top up via the filling loop if needed, following the manual.
• Call a professional: A certified technician should clean the burner, check the flame sensor signal, verify gas inlet pressure, and test the gas valve modulation coil and control board.