Display shows COL or COOL, heater is off and spa indicates water is below acceptable temperature range.

Safety first: Ensure the spa is filled and that all drain plugs are installed to prevent leaks during heating.

• Check set temperature and mode: Verify that the set temperature is above current water temperature and that the spa is not in economy or sleep mode that limits heating.
• Allow time to heat: After a refill with cold water, it can take many hours to reach set temperature. Monitor the temperature rise.
• Verify heater operation: Listen for relay clicks and feel the heater tube after 15–20 minutes of operation; it should be warm but not excessively hot.

If COL persists and the water does not warm at all, further heater and sensor diagnostics are needed.