Display shows FLO or FL and heater will not run; sometimes intermittent heating.

Safety first: Turn off power at the GFCI breaker before opening unions or removing filters.

• Check water level: Ensure water is at the proper fill line, typically above all skimmer and suction fittings.
• Clean or remove filters: Remove the Marquis filter cartridges and rinse thoroughly from inside out with a garden hose. For testing, run the spa briefly without filters after restoring power. If FLO clears, replace or deep-clean the filters.
• Bleed air from system: With power OFF, slightly loosen the union on the circulation pump or heater inlet until air and then water escapes, then retighten. This removes air locks that can prevent flow.
• Inspect circulation pump: Restore power and feel the circulation pump housing. It should vibrate and you should see water movement in the spa. If the pump hums but does not move water, the impeller may be jammed or the pump may be failing.
• Check flow switch: Locate the flow or pressure switch in the plumbing near the heater tube (often a small tee with two wires). Ensure the wires are firmly connected and not corroded. Do not bypass the switch permanently; it is a safety device.
• Test switch (advanced): With power OFF, disconnect the flow switch leads and test continuity with a multimeter while water is flowing (you may need to temporarily power the spa with extreme caution). The switch should close (show continuity) when there is adequate flow.

If FLO persists after filter cleaning and air bleeding, the circulation pump or flow switch assembly may need replacement.