Jacuzzi pool heater lights but then shuts down with F4 error; may cycle on and off frequently.

Safety first: Disconnect power and shut off gas before opening the heater. Allow the unit to cool.

Steps to correct F4:

• Clean the flame sensor: Locate the flame sensor rod near the burner. Remove it carefully and clean the metal rod with fine steel wool or a Scotch-Brite pad to remove oxidation. Do not sand the ceramic insulator.
• Check sensor position: Reinstall the sensor so that it is fully in the burner flame path as specified in the Jacuzzi service manual.
• Inspect wiring and ground: Ensure the flame sensor wire is intact, not brittle or cracked, and firmly connected to the ignition control. Verify the heater chassis has a solid ground connection.
• Check gas pressure (professional): Low inlet gas pressure can cause a weak flame that the sensor cannot detect. A licensed gas technician should measure and adjust gas pressure to Jacuzzi specifications.
• Replace sensor or control if needed: If cleaning and repositioning do not resolve F4, replace the flame sensor. Persistent issues may indicate a failing ignition control board.

If you are not comfortable working around gas and ignition components, contact a qualified Jacuzzi pool heater technician.