Furnace shows F28, repeated ignition attempts with clicking but no flame, then lockout

Critical safety warning: If you smell gas, do not attempt to reset the furnace. Ventilate the area and contact emergency gas services.

• User checks: Ensure the gas meter is on, credit is available (if prepay), and the appliance gas isolation valve is fully open.
• Reset after gas interruption: If gas supply was recently restored, there may be air in the line. Follow the manufacturer’s reset procedure and allow several ignition attempts to purge air, under supervision of a gas technician.
• Electrode inspection: A technician should inspect and clean or replace the ignition electrode and cable.
• Gas valve and pressure: A gas engineer must verify inlet pressure, gas valve operation, and burner pressure. Replace the gas valve if it does not open when energized.