Furnace shows F4, no flame established, ignition clicks may be heard but burner does not light

Critical safety warning: Work on gas components must only be carried out by a licensed gas technician. Do not attempt to adjust or open the gas valve yourself.

• Basic user checks: Ensure the gas supply to the property is available (other gas appliances working) and that the furnace gas isolation cock is fully open.
• Visual inspection: With power off, remove the combustion chamber cover (if you are qualified to do so) and inspect the ignition electrode and cable. Look for cracks in the ceramic, carbon buildup, or damaged insulation.
• Clean electrode: Gently clean the electrode tip with fine emery cloth to remove oxidation. Do not bend the electrode unless you have the manufacturer’s gap specification.
• Check earth connection: A poor appliance earth can affect ignition and flame sensing. Verify that the earth wire is securely connected to the chassis.
• Professional gas checks: A technician should verify gas inlet pressure, gas valve operation, and correct combustion settings using a manometer and flue gas analyzer. If the gas valve coil is open circuit or not being energized, it may require replacement.