Heater attempts to start but shuts down with F02; fan may run briefly.

Safety first: Turn off gas at the shutoff valve and disconnect power before removing burner or ignition components. Risk of fire or explosion if gas leaks are present.

• Verify gas supply: Make sure the external gas shutoff valve is fully open. If other gas appliances are not working properly, contact your gas supplier.
• Inspect the igniter: Remove the burner access panel. Locate the hot surface igniter or spark igniter. Check for cracks, heavy oxidation, or broken wires. Clean light deposits with a soft brush; do not touch a hot surface igniter element with bare fingers.
• Check flame sensor: On many Aquacomfort models, the flame sensor is integrated with or adjacent to the igniter. Gently clean the metal rod with fine emery cloth to remove oxidation, then wipe with a clean dry cloth.
• Inspect burners: Remove the burner tray if accessible. Look for rust, spider webs, or debris in the burner ports and orifices. Clean with compressed air and a soft brush; do not enlarge orifices.
• Reassemble and test: Reinstall burners, igniter, and covers. Restore gas and power. Call for heat and observe through the sight glass: the igniter should glow or spark, then burners should light smoothly. If F02 returns or you smell gas without ignition, shut everything off and call a professional.