Display shows COOL, and the heater is disabled until the water warms slightly.

Safety first: In freezing weather, protect plumbing from freezing by temporarily adding a submersible pump or portable heater in the equipment bay if needed.

• Allow spa to warm: COOL is often informational. As the circulation pump runs, the water will gradually warm, and the code should clear automatically.
• Check cover and insulation: Ensure the Marquis spa cover is in good condition and sealed to retain heat.
• Verify sensor placement: If the sensor is not properly seated in the heater tube or is exposed to cold air, it may read lower than the actual water temperature. Inspect and reseat as needed.
• Monitor for progression: If COOL persists and the water does not warm, there may be a heater or sensor issue that requires further diagnosis.