Display shows F04 and heater will not fire; may occur immediately when calling for heat.

Safety first: Turn off power and allow the heater to cool completely before opening panels. Overheating can damage internal components.

• Verify water flow: Ensure the pump is running at full speed and that the filter is clean. Check that all valves to and from the heater are fully open and that any bypass is correctly adjusted.
• Bleed air from system: If the heater is installed above the pool, air pockets can form. Use any bleed valves on the heater manifold or nearby plumbing to release trapped air with the pump running.
• Inspect high limit switches: Locate the high limit thermostats on the water manifold (usually two disc-type sensors). Check for loose or corroded connections. Do not bypass these safety devices.
• Descale heat exchanger (advanced): Heavy scale can cause localized overheating. A professional can perform a chemical descaling of the copper or cupronickel heat exchanger. Homeowners should not circulate acid through the heater without proper training and equipment.
• Reset and test: Some models have manual-reset high limits; press the reset button if present after the unit has cooled. If F04 recurs quickly, discontinue use and schedule service.