Safety first: Turn off power at the GFCI before removing filters or opening the equipment bay.

Clean or remove filters: Remove all Sunrise Spas filter cartridges. Rinse thoroughly with a high-pressure hose from top to bottom. For testing, run the spa with filters removed for 10–15 minutes. If FLO clears, replace filters if they are older than 1–2 years.
Bleed air from circulation pump: Locate the small circulation pump in the equipment bay. Slightly loosen the highest union fitting until air hisses out and water begins to drip, then retighten.
Check water level and valves: Ensure the water level is correct and all slice valves or gate valves around the pump and heater are fully open.
Test pressure/flow switch: With power off, locate the pressure/flow switch on the heater manifold. Inspect wiring for corrosion or loose connections. With a multimeter, verify continuity when the pump is running (technician level).
If FLO persists: The circulation pump or pressure switch may need replacement. Continued operation with low flow can damage the heater.

Display shows FLO or FL, and the heater will not run.

Possible Causes