Spa will not heat, water stays cold even though pumps run and set temperature is high.

Safety first: Turn off power at the GFCI breaker before opening the equipment bay.

• Confirm settings: Verify the set temperature on the Aquaterra topside control is above the current water temperature and that the spa is not in economy mode (if equipped).
• Check for heat indicator: Look for a heat icon or indicator light on the control panel when the spa should be heating. If no indicator appears, the control may not be calling for heat.
• Inspect heater connections: In the equipment bay, check that the heater terminals are tight and free of corrosion or burn marks.
• Feel heater tube: After restoring power and allowing a 10–15 minute heating cycle, carefully feel the outside of the heater tube. It should be warm if operating. If it remains cold, the heater is likely not energizing.

Testing heater voltage and resistance requires a multimeter and should be done by a qualified technician. A failed heater element or relay usually requires replacement of the heater assembly or control board.