Heater locks out with F03; unit may start then quickly shut down without heating water.

Safety first: Turn off gas at the shutoff valve and disconnect power before opening the burner compartment. Gas leaks and ignition faults can cause fire or explosion.

• Verify gas supply: Ensure the gas valve feeding the Thermeau heater is fully open. Confirm other gas appliances (if present) are working to rule out a supply issue.
• Inspect igniter: Remove the access panel and locate the igniter assembly. Look for cracks, heavy oxidation, or broken wiring. A damaged igniter should be replaced with the correct Thermeau igniter model.
• Clean flame sensor: The flame sensor rod can become coated with oxidation. Gently clean it with fine steel wool or emery cloth, avoiding bending the rod. Reinstall and ensure it is properly positioned in the burner flame path.
• Check burner area: Inspect the burner tray and orifices for rust, spider webs, or debris. Clean carefully with a soft brush and vacuum; do not enlarge orifices.
• Reset and test: Restore gas and power, then call for heat. Observe through the sight glass: the igniter should glow or spark, then burners should light smoothly. If F03 persists, the gas valve or control board may require professional diagnosis.