Furnace locks out with E12, often after ignition; may indicate flame instability.

Safety: Turn off power and gas before removing the combustion cover. Gas adjustments must only be done by qualified personnel.

• Check other gas appliances: If other gas appliances are weak or not working, contact your gas supplier.
• Inspect burner and electrodes: Remove the burner assembly if accessible. Clean burner ports with a soft brush and compressed air. Clean the flame sensor electrode with fine emery cloth.
• Check earth connection: Ensure the furnace is properly earthed and that the earth wire is securely connected to the chassis and supply.
• Observe flame (through sight glass): On restart, observe the flame. A lifting, noisy, or very small flame suggests gas supply or burner issues that require a technician.
• Reset: After cleaning and checks, reset the furnace. If E12 recurs, a professional must check gas valve pressures and control board.