Heater locks out with F3 error after ignition; burner lights briefly then goes out.

Safety first: Isolate electrical power and turn off gas at the appliance valve. Only a qualified gas engineer should adjust gas components.

• Check earth bonding: Ensure the heater chassis is properly earthed; poor earthing can weaken flame rectification signals.
• Inspect flame sensor: Locate the flame sensor (often combined with the ignition electrode on Certikin units). Clean the metal rod gently with fine emery cloth to remove oxidation. Do not damage the porcelain insulator.
• Check wiring: Inspect the flame sensor lead and connectors for corrosion, loose connections, or heat damage. Reseat all plugs on the ignition control module.
• Ventilation and flue: Confirm that the heater has adequate ventilation and that the flue is not partially blocked, which can cause unstable flames.
• Restart: Restore power and gas, then attempt to restart. If F3 persists, stop further attempts.

Ongoing F3 errors require professional testing of gas pressures, burner performance, and the ignition control module.