Display shows HI or HIGH TEMP and unit shuts down; water may feel very warm near heater outlet.

Safety first: Turn off power and allow the system to cool before working on the heater.

• Check setpoint: Verify the desired water temperature on the AquaCal control. Reduce the setpoint to a safe level (e.g., 78–82°F for pools, 100–104°F for spas).
• Verify actual water temperature: Use a reliable pool thermometer away from the heater outlet to confirm the real water temperature.
• Inspect circulation: Ensure the pump is running and that return jets are strong. Clean the filter and check for closed valves that could reduce flow.
• Check sensor accuracy: If the display shows a much higher temperature than actual, the water temperature sensor may be faulty. Inspect wiring and replace the sensor if needed.
• Control board evaluation: If a new sensor does not resolve the HI error and readings remain incorrect, the control board may require replacement by a qualified technician.